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JNITED STATES DEPARTMENT OF AGRICULTURE 
BULLETIN No. 957 

Contribution from the Bureau of Plant Industry 
WM . A. TAYLOR, Chief 



Washington, D. C. 



PROFESSIONAL PAPER 



February 6, 1922 



INVESTIGATIONS OF THE WHITE-PINE 
BLISTER RUST 



By 



PERLEY SPAULDING, Pathologist, 
Office of Investigations in Forest Pathology 



CONTENTS 



Page 

Scope of the Investigations l 

Origin and Distribution of Cronartium 

ribicola '3 

Hosts of Cronartium ribicola . ..... H 

Life History of Cronartium ribicola . . 24 



Page 
Overwintering of Cronartium ribicola . . 68 
Important Dates in the Life History of 

Cronartium ribicola 72 

Control of the White-Pine Blister Rust . 73 
Literature Cited 90 




WASHINGTON 

GOVERNMENT PRINTING OFFICE 

1922 



■-;■■ ■■ '-. ; ., -» ■■•■■■■ 



LIBRARY OF CONGRESS 

FEB £31922., 

DOCUMENT* ii*v iilON 



X 



UNITED STATES DEPARTMENT OF AGRICULTURE 



BULLETIN No. 957 

at iSb 

„Egp* Contribution from the Bureau of Plant Industry **yy 

*>Wfwl WM. A. TAYLOR, Chief -JTWV V%J"d- 




Washington, D. C. 



PROFESSIONAL PAPER 



February 6, 1922 



INVESTIGATIONS OF THE WHITE-PINE BLISTER 

RUST. 

By Perley Spaulding, Pathologist, Investigations in Forest Pathology. 



CONTENTS. 



Page. 

Scope of the investigations 1 

Origin and distribution of Cronartium ribi- 

cola 3 

Hosts of Cronartium ribicola 11 

Pines infected and likely to becom e 

infected 11 

Inoculations of Cronartium ribicola on 

pines 12 

Species of Kibes that have been infected 

naturally 14 

Inoculations of Cronartium ribicola on 

Ribes 16 

Susceptibility of Ribes species and varie- 
ties to Cronartium ribicola 23 



Life history of Cronartium ribicola 

The Peridermium stage on pines 

The Cronartium stage on Ribes 

Overwintering of Cronartium ribicola 

Important dates in the life history of Cronar- 
tium ribicola 

Control of the white-pine blister rust 

Significant factors which determine con- 
trol 

Experiments in control in Europe 

Experiments in control in North America 
Status of the control of white-pine blister 

rust 

Literature cited 



Page 
24 
24 
40 
68 



76 



90 



SCOPE OF THE INVESTIGATIONS. 

In a previous publication (131) * the writer collected data on the 
more practical aspects of the white-pine blister rust, as presented in 
European literature. 

Experience has shown that the white-pine blister rust has come to 
North America to stay and that most careful and searching investi- 
gations must be maintained to enable us to cope with it at all success- 
fully. Investigations in Europe have been carried on in a desultory 
way for 35 years. In North America they were begun less than 15 
years ago. Really intensive work has been in progress for only 
about 5 years. Considerable new experimental work has been done 
in Europe since the appearance of this earlier publication. At various 
times since 1911 some of the more salient results of the investigation 
of Cronartium ribicola Fischer and the disease caused by it have been 
published (132 to 148, 180). During the years 1915 to 1919, inclu- 
clusive, publication has fallen far behind the investigations. 

1 The serial numbers in parentheses refer to " Literature cited" at the end of this bulletin. 
46103- 21— Bull. 957 1 



2 BULLETIN 967, U. S. DEPARTMENT OF AGRICULTURE. 

No attempt has ever been made to collate and summarize the 
results of all the experimental work. The mass of information is so 
large and so scattered that it is nearly impossible for a single indi- 
vidual,, even now, to learn what has been done. This condition is 
certain to become more and more acute as the extensive and intensive 
researches now under way progress. Various States are taking up 
work on this disease, and the multiplication of workers can only 
result in confusion and unnecessary duplication of work unless the 
ascertained data are arranged and made available for all. This 
bulletin aims to present the available information so that the gaps 
in our knowledge may be readily perceived and new investigations 
planned to the best advantage. 

The work of the Office of Investigations in Forest Pathology has 
been conducted under the direction and advice of the writer by the 
following persons: In 1915, G. F. Gravatt and Dr. G. R. Lyman; 
in 1916, Dr. R. H. Colley, G. F. Gravatt, and Miss M. W. Taylor; 
in 1917, Dr. R. H. Colley, G. B. Posey, G. F. Gravatt, Rush P. Mar- 
shall, and Miss M. W. Taylor; in 1918, Drs. R. H. Colley, H. H, York, 
L. H. Pennington, L. O. Overholts, A. S. Rhoads, T. C. Merrill, 
W. H. Snell, D. M. Benedict, and Miss M. W. Taylor; in 1919, Drs. 
R. H. Colley, H. H. York, and L. H. Pennington, D. M. Benedict, 
J. E. Lodewick, W. H. Snell, P. R. Gast, Miss A. E. Rathbun, and 
Miss M. W. Taylor. Dr. G. G. Hedgcock made a comparative study 
of Cronartium occidentals and C. ribicola on Ribes on Block Island, 
R. L, in 1919. 

The work of Dr. L. H. Pennington, D. M. Benedict, and J. E. Lode- 
wick, in 1919, was maintained in formal cooperation with the New 
York State College of Forestry at Syracuse University. 

The endeavor has been to show plainly in this bulletin who did 
each piece of work without entering into details to an objectionable 
extent. 

The writer thanks the following people for unpublished data which 
have been placed at his disposal: Mr. W. A. McCubbin, formerly of 
Canada; Dr. Ed. Fischer, of Switzerland; Dr. A. B. Borthwick, of 
Scotland; Mr. A. D. Cotton, of Kew Gardens, England; Prof. L. 
Mangin, Museum of Natural History, Paris; Prof. F. K0lpin Ravn 
and Mr. J. Lind, of Denmark; Dr. L. O. Kunkel and Mr. W. Stuart 
Moir, of this country. 

The writer and his collaborators are indebted for material for 
experimental use to the Arnold Arboretum of Harvard University; 
the Dominion of Canada Central Experimental Farms; the Park 
Board of Rochester, N. Y.; the Conservation Commission of the 
State of New York; the Office of Horticultural and Pomological In- 
vestigations and the Forest Service, of the United States Department 
of Agriculture. The Office of Blister-Rust Control has contributed 



WHITE-PINE BLISTER RUST. 3 

data, especially toward that appearing in the chapter on control, Table 
V, and the list of Ribes species infected in the different States. Much 
difficulty has been encountered in getting satisfactory translations 
of articles published in the Japanese, Russian, Dutch, Swedish, Nor- 
wegian, and Danish languages. Dr. E. P. Meinecke has very kindly 
translated most of the Scandinavian and Danish articles. Mr. Rush 
P. Marshall and Miss M. W. Taylor, have aided in checking and col- 
lating the extensive data here presented. 

In this bulletin the behavior of Cronartium ribicola is given with 
considerable detail. So far as is now known, it agrees essentially 
with the Uredinales in general in its life history and physiology. 
This is the first species of Cronartium to be very intensively investi- 
gated, and as a representative of this important group of forest-tree 
fungi, a detailed knowledge of its life history must form the basis 
for the institution of new methods of management of white-pine 
forests. 

ORIGIN AND DISTRIBUTION OF CRONARTIUM RIBICOLA. 

Some writers (76, 90) have believed that Cronartium ribicola went 
to Europe from America on Ribes aureum, that host being associated 
with it (but not exclusively) in the earlier discoveries of the disease 
in Europe. Magnus, who was of this opinion at first (90), seems to 
have completely rejected this theory and now believes that the dis- 
ease came from western Siberia and the Swiss Alps, where it is sup- 
posed (26, 39, 40, 93, 130, 174) to have been endemic on Pinus cembra. 
In 1842 Klotsch issued in the exsiccatse entitled "Herbarium vivum 
mycologicum, No. 490," a specimen labeled "Uredo ribicola" col- 
lected by Lasch at Driessen. Specimens have not been seen by the 
writer, and there is some uncertainty whether or not this is actually 
the uredinial stage of Cronartium ribicola. Sydow (155) gives it as 
a synonym of C. ribicola, but he is the only author known to the 
writer who does so. 

Cronartium ribicola was first certainly found by Dietrich (27) in 
the Baltic provinces of Russia in 1854. He found it upon Ribes 
nigrum, R. u rubrum," and R. " palmatum," and also upon Pinus 
strobus, although at that time it was not known that the two forms 
were stages of a single fungus. So far as can be determined from 
scientific literature, it was not again noted until 1861 in Finland (81), 
1865 in East Prussia (76), and 1869, when Eriksson (31) found it in 
Sweden on Ribes nigrum, and Hisinger (54) noted the first outbreak 
on pines in Finland. It had attacked Pinus strobus trees 30 years 
old and killed them. In 1883 Rostrup (115) reported an outbreak 
in Denmark on Pinus strobus trees 20 to 30 years old. It was evi- 
dently then generally spread over that country. Still later, Klebahn 
(62, 63) and Tubeuf (169) record it as generally distributed over 



4 BULLETIN 957, U. S. DEPARTMENT OF AGRICULTURE. 

Germany, and soon it had spread over the northern countries of 
Europe (91, 92, 93). It seems to the writer that this was an instance 
of the introduction of a foreign disease into Europe and its destruc- 
tive spread over most of that continent (39, 40) . The fact that Pinus 
strobus had been grown extensively in Europe since its introduction 
there in 1705 (5), but was not known to have this disease until about 
1855, in the light of our experience with this and other introduced 
plant diseases in North America, shows that this was a new disease 
which had reached Europe probably years before its discovery. 




Fig. 1. — Outline map of the Old World, showing the approximate distribution of Pinus cembra (oblique 
hatching) and of its variety pumila (vertical hatching) together with the known distribution of Cronar- 
tium ribicola (black dots). Tree distribution furnished by the Forest Service, United States Department 
of Agriculture. 

It is not certain that Cronartium ribicola is a native of the Swiss 
Alps (174). Schellenberg (123), in 1903, found Cronartium ribicola 
on a single 15-year branch of a tree of Pinus cembra about 200 years 
old in the Engadine Valley, Switzerland. He believed that the 
fungus was native there on this host. Yet this is the first known 
finding of the fungus on pine in that region. Ribes diseased with it 
were found there in 1895 (39, 123), showing it to be established in 
that locality then. It seems to the writer that the circumstances 
point plainly to the fungus having come into Switzerland some time 
previously, and that it is not endemic there; else it would have been 



WHITE-PINE BLISTER RUST. 5 

found much earlier, and more of it would have been found since 
then. Fischer (40) found the disease in 1915 spreading into western 
and northern Switzerland from without. 

It is generally supposed that Pinus cembra (26, 70, 93, 97, 98, 123) 
is the original pine host of this fungus. Figure 1 shows the distri- 
bution of P. cembra and its variety pumila in Europe and Asia. 
Cronartium ribicola is reported from Asia as follows: In 1879, from 




Fig. 2.— Outline map of the United States, showing the known distribution of Cronartium ribicola and 
C. occidentale in North America to January 1, 1920. Localities for Cronartium occidentale are shown by 
black squares in the Pacific coast and Rocky Mountian regions, the easternmost point being in western 
Kansas . This is where it was found in 1S92, but it has not been seen there since. Localities for C. ribicola 
are indicated by double cross hatching and black dots, nearly all being north of the Potomac and Ohio 
Rivers and east of the Mississippi River. Four points in southwestern Minnesota, eastern South Dakota, 
and northern Iowa were found to be due to diseased nursery stock which was shipped in . It is belie ved 
that the disease now has been eradicated in these outer western localities. The natural distribution of 
the eastern white pine is shown in the large cross-hatched area mostly east of the Mississippi River . The 
cross-hatched areas shown on the western half of the map indicate the known distribution of the western 
white pines. The pihon pines range as far north as southern Idaho but at altitudes different from those 
of the white pines. Cronartium ribicola is limited to the eastern white-pine area and was not known in 
North America until 1906. In most places where now found it has been traced to diseased imported 
white-pine stock. Cronartium occidentale is limited to that part of the western white-pine area in which 
pinon pines are native, where it appears also to be native. The two fungi are separated by a strip of 
prairie country about 500 miles wide. Distribution of the pines furnished by the Forest Service, United 
States Department of Agriculture. Distribution of Cronartium occidentale furnished by Messrs. Bethel 
and Posey, of the Offices of Investigations in Forest Pathology and of Blister-Rust Control, respectively. 

Bolschaja Inja River (131, 161); also from Tomsk and Minusinsk, 
Siberia (131). Quite recently it has been reported from Sakhalin 
Island and from Sapporo, Japan (156). Tulasne (175) in 1854 re- 
ported a Cronartium on Ribes, probably in India. Clinton has 
announced (13) the finding of Cronartium ribicola on dried herbarium 
specimens of Ribes collected by Wilson in the western part of the 
Province of Hupeh, China, in 1900. 



6 



BULLETIN 957, IT. S. DEPARTMENT OF AGRICULTURE. 



To sum up briefly: Pinus cembra, the probable original pine host, 
ranges across northern Asia; and the fungus is reported from western 
eastern, and central Asia, in some places where it may easily be 
endemic. 

In North America, Cronartium ribicola was first found in 1906 at 
Geneva, N. Y. (3, 150). Later findings have indicated that it was 
here in the Northeastern States as early as 1898 (108, 136, p. 6). It 
might have been in North America a few years, but not many, before 
that date. This is supported by Clinton (13), who unsuccessfully ex- 
amined specimens of Bibes which are in some of the larger herbaria 
of the eastern part of this country. The writer has supplemented 



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Fig. 3.— Outline map of the northeastern part of the United States, showing (by black dots) the known 
distribution of white-pine blister rust in North America to and including 1909. 

Clinton's work by examining the Kibes specimens in several addi- 
tional herbaria. These include the Pringle herbarium at the Uni- 
versity of Vermont and the local collections of the University of 
Vermont; of Dartmouth College; of President Ezra Brainerd, of 
Middlebury College; of Mr. C. A. Weatherby, of East Hartford, 
Conn. ; and of Mr. C. H. Bissell, of Southington, Conn. The most 
notable herbarium examined was that of the Boston Society of 
Natural History, which contains many New England collections 
made in the early years of the nineteenth century. Moreover, such 
keen fungus collectors as Farlow, Seymour, G. P. Clinton, Peck, 
Ellis, George Clinton, Stewart, and many others, never collected 
Cronartium ribicola until 1906, showing that it is a recent immi- 
grant. Since 1909, when it was first found in North America on 
white pines, Cronartium ribicola has spread until it is firmly estab- 



WHITE-PINE BLISTER RUST. 7 

lished in New England, New York, Wisconsin, Minnesota, and 
Canada. 

It has been known since 1892 that there was a fungus on Kibes in 
the West much resembling Cronartium ribicola, but until 1917 its 
alternate stage on pines was unknown. In that year the Office of 
Investigations in Forest Pathology began work upon this western 
fungus, which was soon found to have an alternate stage on Pinus 
edulis and P. monopJiylla in Colorado and Arizona (50, 114) and was 
named Cronartium occidentale. Its distribution and that of C. ribi- 
cola as known to January 1, 1920, is shown on the map (fig. 2). See 
figures 3 to 12 for the progress of C. ribicola by years from 1909 to 




Fig. 4.— Outline map of the northeastern part of the United States, showing (by black dots) the known 
(cumulative) distribution of white-pine blister rust in North America to and including 1910. 

1918, inclusive. Cronartium occidentale is found in localities where 
it could hardly be an introduction, as the Ute Indian Reservation in 
southwestern Colorado, where it was found by Bethel in 1897; also 
in the Mesa Verde region, where no cultivated Ribes or pines have 
ever been introduced. Ribes aureum is native in the Rocky Mountain 
region and is a favorite host for Cronartium occidentale as well as 
C. ribicola. 

Since Ribes aureum was intimately associated with Cronartium 
ribicola in its earlier known occurrences in Europe, an inquiry has 
been made into the possibility of the fungus being American in origin 
and its being introduced into Europe on R. aureum when that plant was 
first sent there. The facts thus far determined are 2 that R. aureum 



2 Spaulding, Perley. Ribes aureum not an original host of Cronartium ribicola. In manuscript. To 
be published in Phytopathology. 



8 



BULLETIN 957, TJ. S. DEPARTMENT OE AGRICULTURE. 



is definitely stated to have been introduced into England (the proba- 
ble first point of introduction in Europe) in 1812 by Thomas Nuttall. 
Evidently Nuttall collected the seeds of this plant while on a botanical 




Fig. 5.— Outline map of the northeastern part of the United States, showing the known distribution of 
white-pine blister rust in North America to and including 1911. 




Fig. 6.— Outline map of the northeastern part of the United States, showing (by black dots) the known 
distribution of white-pine blister rust in North America to and including 1912. 

trip in 1811 with John Bradbury up the Missouri River to a point in 
the eastern part of Mercer County, N. Dak. His material must have 
been shipped quite promptly to England, as Fraser's nursery at 



WHITE-PINE BLISTER RUST. 



9 



Chelsea, London, in 1813, offered for sale plants " collected in Upper 
Louisiana and principally on the River Missourie, North America." 
There is reason to believe that these were Nuttall's plants. The 





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Fig. 7.— Outline map of the northeastern part of the United States, showing (by black dots) the known 
distribution of white-pine blister rust in North America to and including 1913. 




Fig. 8.— Outline map of the northeastern part of the United States, showing (by black dots and cross 
hatching) the known distribution of white-pine blister rust in North America to and including 1914. 
The cross hatching in this and the following maps indicates areas which are generally infected. 

plant of interest to us is listed as Ribes longiflorum, a new species 
from the Missouri. This is now known as R. odoratum Wendland, 
which is the common form generally cultivated in the eastern United 



10 



BULLETIN" 957, U. S. DEPARTMENT OF AGRICULTURE. 



States under the name R. aureum. This appears to be the form most 
common in cultivation in Europe, but the true R. aureum of the 
Rocky Mountain region and the plains of the Columbia was evi- 
dently introduced there soon after R. odoratum, if not at about the 
same time. Pursh may have referred to living plants of true R. 
aureum but we have no means of determining this. Lindley in 1828 
made the new species R. tenuiflorum (now R. aureum) and says 
"Clt. 1812," evidently meaning "cultivated in 1812." He had this 
so definitely distinguished from our R. odoratum that his statement 
is fairly conclusive that R. aureum actually was introduced into 
England the same year as was R. odoratum. The agent introducing 




Fig. 9.— Outline map of the northeastern part of the United States, showing (by black dots and cross 
hatching) the known distribution of white-pine blister rust in North America to and including 1915. 

R. aureum can not be determined beyond question. There appears 
to be little doubt, because of the difficulty of communication at that 
time, that both plants were carried from their native regions to the 
eastern part of this country in the form of seeds. Cuttings may have 
been sent to Europe but it is more likely that seeds were sent. Seeds 
would not be likely to transmit a rust which does not attack the 
fruit. It does not seem possible that a fungus like Oronartium 
ribicola could be carried to Europe on these plants without becoming 
established in the Eastern States. Moreover, these species of Eibes 
apparently were introduced into Great Britain first. The fungus 
was not known in Great Britain until 1892, long after it was prevalent 
in other northern European countries. It appears that Oronartium 
ribicola was carried to Great Britain on infected white pines from 
northwestern Germany. The evidence appears to show that Cro- 



WHITE-PINE BLISTER RUST. 11 

nartium ribicola attacked Riles aureum and R. odoratum after they 
reached Europe. Their susceptibility also indicates that they are 
not original hosts of the fungus. 

Summing up the evidence available, it appears (1) that Cronartium 
ribicola is Asiatic in origin; (2) that it spread in the early 1800's 
into western Russia, whence it eventually spread well over Europe 
(41) ; (3) that it was brought to North America in young trees of 
Pinus strobus; and (4) that comparative studies (50) show that 
Cronartium occidentale is distinct. 

HOSTS OF CRONARTIUM RIBICOLA. 

Pines Infected and Likely to Become Infected. 

Cronartium ribicola has attacked 11 of the white pines in the 
countries, Provinces, and States indicated in the following list : 

Pinus aristata Engelmann in England (20a). 

ayacahuite Ehrenb. in Scotland and England. 3 

cembra L. in Russia (58, 93), Switzerland (39, 40, 123, Germany (72, 77, 174, 177), 
U. S. A. (Mass., Minn.). 4 

excelsa Wall, in Denmark (120), Germany (99), U. S. A. (Mass.). 

fiexilis James in Germany (173), Sweden, 5 U. S. A. (Mass., Minn., Iowa). 

horaiensis Sieb. and Zucc. in Sweden. 5 

lambertiana Douglas in Belgium (101), Germany (62, 173). 

monticola Douglas in Belgium (101), England (79), Germany (70). 

parviflora Sieb. and Zucc. in U. S. A. (Mass.). 

pence Gris. in Germany (173). 

strobus L. in Austria-Hungary (178), Belgium (70), Denmark (81, 115, 117, 119, 
120), Finland (54, 83), Switzerland (40), France (70), Germany (70), Great 
Britain (111), Holland (70), Ireland (42), Norway (70), Russia (27, 58, 120), 
Siberia (161), Sweden (131 5 ), Canada— Ontario (23, 56) and Quebec (107, 
121), U. S. A. (Conn., Ind., Iowa, Maine, Mass., Mich., Minn., N. H., N. J., 
N. Y., Ohio, Penn., R. I., S. Dak., Vt., Va., Wis.). The fungus has occur- 
red in a number of these States only on diseased pines shipped from outside 
points. 

In every case the disease attacked these pines naturally in out- 
break areas of Europe and North America and is not known to attack 
any of the pitch pines, although some of them have been present in 
infected areas. Whenever the other white pines are continuously 
exposed to the fungus they will be likely to develop the disease. The 
blister rust was first found on the different species of pines as follows : 

Pinus strobus. Russia in 1854. Pinus ayacahuite. Great Britain in 1908. 
lambertiana. Germany in 1887. fiexilis. Germany in 1914. 

cembra. Russia in 1890. pence. Germany in 1914. 

monticola. England in 1898. parviflora. United States in 1916. 

excelsa. Denmark in 1902. horaiensis. Sweden in 1920. 

aristata. England in 1907. 

3 Communicated in a private letter from Dr. A. B. Borthwick, of Scotland. 

4 The statements concerning occurrences in North America are based on records and specimens in the 
Office of Investigations in Forest Pathology. 

5 Trivate letter from W. Stuart Moir, Office of Blister-Rust Control. 



12 BULLETIN 957, U. S. DEPARTMENT OF AGRICULTURE. 

Sudworth (154) recognizes eight species of the white pines (exclu- 
sive of the pinon pines) for North America (fig. 2) but does not treat 
those of the Old World. Shaw (126), who treats the pines of the 
world, also recognizes eight North American species of white pines. 
He is, therefore, taken as the authority for the pines in this bulletin. 
The white pines of the world are grouped by Shaw as follows : 

Genus Pinus. 

Section Haploxylon. 
Subsection Cembra. 
Group I. Cembrse. 

koraiensis +. 

cembra +• 

albicaulis. 
Group II. Flexiles. 

fiexilis -{-. 

armandi. 
Group III. Strobi. 

ayacahuite (or strobiformis) +. 

lambertiana +. 

parviflora +• 

pence +• 

excelsa +. 

monticola +. 

strobus +. 
Subsection Paracembra. 

Group IV. Cembroides — pinon pines. 
Group V. Gerardianae — pinon pines. 
Group VI. Balfourianse. 

balfouriana. 

aristata-\-. 

In examining the above synopsis, note the grouping of the known 
susceptible species (which are indicated by +) especially in the first 
three groups which make up the subsection Cembra. Investigations 
of outbreak areas in Europe where the various species of pine have 
been present might yield on this point most interesting and valuable 
information which can be obtained in no other way. 

Inoculations of Cronartium Ribicola on Pines. 

Klebahn (68, 71) appears to be the first European investigator 
who has inoculated pines with Cronartium ribicola and who has pub- 
lished his results. He inoculated young Pinus strobus trees with 
pycnospores, but with no success (70, p. 387). Inoculations made by 
him with sporidia in 1888 were rendered worthless because the pines 
were probably infected naturally before the test was made (71). On 
August 27, 1903, Klebahn (71) made inoculations on two young 
Pinus strobus trees by suspending telia-bearing leaves of Ribes nigrum 
above the trees and covering them with a large bell jar. On June 19, 



WHITE-PINE BLISTER RUST. 



13 



1904, the pines were examined. At that time some of the new shoots 
bore abnormal leaves of a juvenile type singly, instead of in fives. 
The new twigs were abnormally pale in color. Many of the leaves 
of the growth of the previous year were spotted with yellow through 
their entire thickness. These yellow spots were especially plentiful 
near the base of the leaves. Microscopic examination showed abun- 
dant mycelium to be present in the yellow areas. Later pycnia devel- 
oped on the twigs near the bases of the spotted leaves. 




Fig. 10.— Outline map of the northeastern part of the United States, showing (by black dots and cross 
hatching) the known distribution of white-pine blister rust in North America to and including 1916. 

In 1914, Tubeuf (174) inoculated trees of Pinus strobus, P. lam- 
bertiana, P. cembra; P. cembroides, P. excelsa, P. pence, P. parviflora, 
P. Jiexilis, and P. montezumae with sporidia of Cronartium ribicola 
under controlled conditions. In 1917, aecia were produced on some 
of the P. strobus. Yellow spots were produced on the leaves of P. 
lambertiana, but no further development of the fungus occurred. 
Spots which were doubtfully caused by the fungus were noted on P. 
cembroides. No other species became infected. Infections were 
produced directly on the juvenile leaves, on mature leaves, and 
through the epidermis of the lengthening buds of the young shoots. 

In North America, the writer seems to have been the first to 
inoculate successfully young Pinus strobus trees with sporidia (133, 
134). The inoculations were made both with and without wounds 
in the young bark, telial columns being used for inoculum. Pycnial 
drops were produced by one tree unwounded and by one which was 



14 



BULLETIN 957, U. S. DEPARTMENT OF AGRICULTURE. 



wounded. At this stage slugs ate the infected bark and prevented 
further development of the fungus. In 1916 and in 1917, 150 
seciospore inoculations were made on leaves, on twigs of various agesi 
and on branches of P. strobus trees up to six years. No infections 
have resulted. 

Clinton and Miss McCormick (14, 15), have published details of 
successful inoculations, through the leaves, with sporidia on P. 
strobus. Inoculations were unsuccessful upon leaves of P. excelsa, 
P. Jlexilis, P. Icoraiensis, and P. cembra; also on the pitch pines P. 
resinosa, P. sylvestris, P. densiflora, and P. austriaca. Yellow spots 





p^ 7 






1 




~x 








1 



Fig. 11. — Outline map of the northeastern part of the United States, showing (by black dots and cross 
hatching) the known distribution of white-pine blister rust in North America to and including 1917. 

have been secured on leaves of P. lambertiana, P. pinea, and P. 
sabiniana. 

Cross-inoculations that are known to have been successful up to 
July 1, 1920, are shown in Plate I. 

Species of Ribes That Have Been Infected NaturaJly. 

In Europe and North America, where extensive outbreaks of 
Cronartium ribicola have occurred, a considerable number of species 
of Ribes have been found naturally infected by the fungus. Prac- 
tically all of the cultivated species and most of the wild ones take the 
disease in every extensive outbreak area. More species have been 
found infected in Europe than in North America, because outbreaks 
have been discovered there in botanical gardens, parks, and nurseries 



WHITE-PINE BLISTER BUST. 15 

where extensive collections of the different species of Ribes were 
located, and hence more species have been subjected to attack. 

In Europe the following species have become infected naturally: 
Ribes aciculare Smith, R. affine H. B. K., R. alpinum L., R. ameri- 
canum Mill., R. aureum Pursh, R. biebersteinii hybrid, R. bracteosum 
Douglas, R. cynosbati L., R. divaricatum Douglas, R. glandulosum 
Grauer, R. gordonianum hybrid, R. Mrtellum Michx., R. irriguum 
Douglas, R. menziesii Pursh, R. missouriense Nuttall, R. multiflorum 
Kit., R. nigrum L., R. niveum Lindl., R. odoratum Wendl., R. oxya- 
canthoides L., R. petraeum Wulf., R. reclinatum L., R. rotundifolium 
Michx., R. rubrum L., R. sanguineum Pursh, R. setosum Lindl., R. 
triste Pallas, R. vulgare Lam. 

The names of species of Ribes of North America are based on the 
treatment of the North American species by Coville and Britton in 
f North American Flora" (22) and of species of the other continents 
on that of Janczewski in "Monographie des Groseilliers, Ribes L." 
(60). For the sake of convenience the currants and gooseberries are 
kept in the single genus Ribes. 

As recently as 1914 (136) the fungus had not been found in North 
America attacking any wild species of Ribes. Since then it has 
been found attacking an increasing number of wild Ribes, as addi- 
tional species are subjected to infection by the spreading and multi- 
plication of the known outbreak areas and as new ones are dis- 
covered. To date it is known to have been found attacking naturally 
the following species of Ribes in the States and Provinces named: 

Ribes americanum (floridum). — Maine, New Hampshire, Vermont, Massachusetts, 
Rhode Island, New York, Wisconsin, Minnesota, and Ontario. 

cynosbati. — Maine, New Hampshire, Vermont, Massachusetts, Rhode Island, 
Connecticut, New York, New Jersey, Wisconsin, Minnesota, and Ontario. 

glandulosum (prostratum) . — Maine, New Hampshire, Vermont, Massachusetts, 
Connecticut, New York, Wisconsin, Minnesota, and Ontario. 

Mrtellum.- — Maine, New Hampshire, Massachusetts, New York, Wisconsin, and 
Minnesota. 

irriguum. — New York. 

lacustre. — Maine, New Hampshire, New York, and Wisconsin. 

missouriense. — New York, Minnesota, and Wisconsin. 

nigrum. — Maine, New Hampshire, Vermont, Massachusetts, Rhode Island, 
Connecticut, New York, New Jersey, Wisconsin, Minnesota, Ontario, and 
Quebec. 

odoratum. — Maine, New Hampshire, Vermont, Massachusetts, Rhode Island, 
Connecticut, New York, Minnesota, and Ontario. 

reclinatum (grossularia). — Maine, Vermont, Massachusetts, New York, Wiscon- 
sin, Minnesota, and Ontario. 

rotundifolium. — New York. 

triste. — Maine, New Hampshire, Massachusetts, Wisconsin, Minnesota, and 
Ontario. 

vulgare (rubrum). — Maine, New Hampshire, Vermont, Massachusetts, Rhode 
Island, Connecticut, New York, Wisconsin, Minnesota, and. Ontario. 



16 



BULLETIN 957, IT. S. DEPARTMENT OF AGRICULTURE. 



Inoculations of Cronartium ribicola on Ribes. 

In 1888, Klebahn (63, 64) made the first known successful inocu- 
lations of this fungus on Ribes. Since that time a number of inocu- 
lations have been made in Europe by Klebahn (65, 66, 67, 68, 69, 
70), Wettstein (70), Rostrup (118, 119, 120), Sch</>yen (124), Eriksson 
(31, 33), Tubeuf (167), Hennings (53), Sorauer (129), Tranzschel 
(92), Neger (100), Ewert (36, 37), Naumann (36), and Jaczewski 
(58, 59). The published accounts of most of these experiments are 
very fragmentary and lack many essential details. In fact the 
attitude of the European investigators seems to have been that of 
mild interest in a new fungus rather than that of intensive study of 
a new parasite and of the destructive disease caused by it. 




Fig. 12.— Outline map of the northeastern part of the United States, showing (by black dots and cross 
hatching) the known distribution of white-pine blister rust in North America to and including 1918. For 
distribution to the end of 1919, see figure 2. 

Since the summer of 1909 the writer and his collaborators have 
made repeated tests under controlled and natural conditions of all 
species and of all varieties of the cultivated species of Ribes that 
could be obtained. In the earlier work complete records were not 
kept, but in the last few years the records were made to cover all points 
likely to be of value. Hundreds of inoculations have been made on 
the more susceptible species to keep the fungus growing in vigorous 
condition, without records being made. It was felt that green- 
house tests alone were not dependable for susceptibility data. 
Therefore, in 1916, a test plat was located upon Block Island, R. I. 
This island lies 12 miles from the nearest projecting point of the 
mainland and 15 miles or more from the main shore line. No white 
pines are on the island and but few cultivated Ribes. It was chosen 



Plate 1. 



Ribes: 



Pinus 




aljoestre 

alfjinum Tnacrojahyllw' 

mericaTium 
ureum 

bracTeosum 

£arrierii 

/ cereum 
coloraci^nsa 

/Culverwellii 




Including Those r 



' European Investigators. 



WHITE-PINE BLISTER RUST. 17 

as the safest place for such work at that time. In midsummer, 1916, 
before the disease was fairly started on the bushes on the island, it 
was found to be pretty generally distributed over New England on 
Ribes, having plainly been widely disseminated there before the 
Block Island experiment was started. Conditions on the island are 
not very favorable for Ribes and are decidedly unfavorable for white 
pines. Pinus nigra var. austriaca is the only pine seen on the island, 
and it occurs only in protected hollows. 

Table I presents the general results of tests made in the green- 
house where inoculations were made with both seciospores and 
urediniospores. A preliminary statement has been published, giv- 
ing the earlier results of this work (147). This table is to be inter- 
preted as follows: 

In the columns headed "Susceptibility," a single cross (X) means slight infection, 
two crosses (XX) mean a medium degree of infection, and three crosses (XXX) mean 
heavy infection. 

There are no means of knowing what degree of susceptibility was indicated by 
the experiments of European and Canadian investigators, whose results are included 
in section 1 of Table I. The foreign experimenters are first listed alphabetically 
under each species of Ribes, then the work done in this country is given in a similar 
manner. The varietal tests (sections 2 to 4 of Table I) are wholly the work of the 
Office of Investigations in Forest Pathology, there being practically no data in 
foreign literature on inoculations of horticultural varieties. 

In numerous cases but a single test has been made as yet; but the 
general behavior of the tested plants in the spread of the fungus, 
the type of its fruiting, etc., during the rest of the season are con- 
sidered in the final estimate of susceptibility. When a single test 
has been made under favorable conditions, it is believed that the 
results are fairly indicative of the susceptibility of the species or 
variety tested. Many tests were made under conditions known to 
be adverse, and the negative results are largely due to this cause alone. 
But these tests are given with the others to give some idea of what 
has been done. Scanty numbers of tests are often due to the Ribes 
stock dying before a second test could be made. This is true of 
many cuttings which made a weak start and did not survive potting. 

In sections 2 to 4 of Table I, relating to the varieties of Ribes, an 
attempt has been made to use varietal names that are intelligible to 
horticulturists as accepted by the American Pomological Society. 
Acknowledgment is made to the Office of Horticultural and Pomo- 
logical Investigations of the United States Department of Agriculture 
for help in this matter. In some cases varietal names are given 
which are considered to be synonyms of others in the list; but in such 
cases the stocks used under the two names were evidently different. 
Varieties and even species of Ribes supplied by nurserymen are often 
other than what they purport to be, and in some cases they are 
mixtures of two or more distinct things; hence, the varietal names can 
not be taken as being absolutely reliable. 
461 03°— 21— Bull. 957 -2 



18 



BULLETIN 951, U. S. DEPARTMENT OF AGRICULTURE. 



Table I. — Results of inoculations made on various species and horticultural varieties of 
Ribes, showing data on susceptibility in the greenhouse and out of doors on Block Island, R.I. 



Species or variety 
and investigators. 



Inocula- 
tions. 



Sec. 1.— Species of 
Ribes. a 



alpestre: 

Gravatt. 
alpinum: 

Klebahn. 

Neger . . . 

Sorauer. . 



Gravatt 

Marshall 

Taylor 

R. alpinum macro- 
phyllum: 

Rhoads 

Spaulding 

R. amarum: 

Taylor 

R. americanum: 

Sorauer 



Gravatt . . . 
Spaulding . 

Taylor 

York 

R. aureum: 
Klebann. . 
Sorauer... 
Tubeuf... 



Gravatt 

Hedgcock 

Marshall 

Spaulding 

Taylor 

R . aureum ' ' palma- 
tum foeminum": 

Spaulding 

R. aurem X recli- 
natum: 

Gravatt 

R. bethmontii: 

Spaulding 

R. bracteosum: 

Gravatt 

R. carrierei: 

Gravatt 

R. cereum: 

Gravatt 

Marshall 

Spaulding 

Taylor 

R. coloradense: 

Spaulding 

Tajdor 

R. cruentum: 

Gravatt 

R. culverwellii: 

Gravatt 

Marshall 

R. curvatum: 

Gravatt 

Marshall 

Spaulding 

Taylor.... 

R. cvnosbati: 

tubeuf 



Infec- 
tion. 



Num- 
ber. 



Gravatt 

Hedgcock 

Lyman 

Spaulding 

a Summary of tests 



Susceptibility. 



In the 
green- 
house. 



XXX 



XX 



XX 



XX 



Out of 
doors. 



XX 



XX 



Species or variety 
and investigators. 



Sec. 1— Species of 
Ribes— Contd. 

R. diacantha: 

Gravatt 

Taylor 

R. divaricatum: 

Rostrup 

Tubeuf 



3 4 

1 1 

2 3 
2 2 

made in foreign countries and in America 



Gravatt 

Spaulding 

Taylor 

R. erytbrocarpum: 

Gravatt 

R. fasciculatum: 

Gravatt 

Merrill 

Spaulding 

Taylor 

R. fasciculatum chi- 
nense: 

Gravatt 

Merrill , 

Spaulding 

R. fasciculatum jap- 
onicum: 

Spaulding 

R. fontanum: 

Colley and Tay- 
lor 

Gravatt 

R. fontenayense: 

Gravatt 

R. futurum: 

Gravatt 

Merrill 

Spaulding 

Taylor 

R. giraldii: 

Gravatt 

R. glandulosum: 

Lyman 

SneU 

Spaulding 

Taylor 

R. glutinosum: 

Gravatt 

Spaulding 

R. gordonianum: 

Gravatt 

Spaulding 

R. hesperium: 

Gravatt 

Merrill 

Taylor 

R. hirtellum: 

Gravatt 

Lyman 

Spaulding 

Taylor 

R. hirtellum X re- 
clinatum: 

Hedgcock 

R. holosericeum: 

Gravatt 

R. inebrians: 

Hedgcock 

R, inerme: 

Gravatt 

Hedgcock 

Spaulding 

Taylor 



Inocula- 
tions. 



Infec- 
tion. 



Num- 
ber. 



Susceptibility. 



In the 
green- 
house. 



XX 



XX 



XX 

XX 

X 

XX 



XX 



} XX 



XX 



X 

XX 
X 

XX 



Out of 
doors. 



XX 



XX 



WHITE-PINE BLISTER RUST. 



19 



Table I. — Results of inoculations made on various species and horticultural varieties of 

Ribes, etc. — Continued. 



Species or variety 
and investigators. 



£^ec. I.— Species of 
Ribes— Contd. 

R. innominatum: 

Spaulding 

Taylor 

R. irriguum: 

Gravatt 

R. lacustre: 

Gravatt .... 

Spaulding 

Taylor 

R. leptanthuni: 

Gravatt 

Spaulding 

Taylor 

R. lobbii: 

Gravatt 

R. malvaceum: 

Gravatt 

Spaulding 

R. malvaceum yiri- 
diiolium: 

Gravatt 

Merrill 

Spaulding 

R. menziesii: 

Gravatt 

Taylor 

R. missouriense: 

Rostrup 

Gravatt 

Lyman 

Spaulding 

Taylor 

R. missouriense X 
reclinatum: 

Gravatt 

R. montigenum: 

Spaulding 

R. multiflorum: 

Rostrup 

Rhoads 

Spaulding 

R.nevadense: 

Gravatt 

Spaulding 

R. nigrum: 

Eriksson 

Ewert 

Hennings 

Klebahn 

Jaczewski 

McCubbin 

Naumann 

Rostrup 

Sorauer 

Tranzschel 

Tubeuf 

Gravatt 

Hedgcock 

Marshall 

Merrill 

Spaulding 

Taylor 

York 

R. nigrum "aconiti- 
folium"! 

Spaulding 

R. nigrum "crisp- 
um": 

Sorauer 

& Slight. 



Inocula- 
tions. 



Infec- 
tion. 



Num- 
ber. 



Susceptibility 



In the 
green- 
house. 



XX 
X 

XX 



Jf> 



}x: 



XX 



XX 



Out of 
doors. 




Species or variety 
and investigators. 



Sec. 1. — Species of 
Ribes— Contd. 

R. nigrum "fascicu- 
latum": 

Rhoads 

R. nigrum "folio 

argentea": 



Inocula- 
tions. 



Infec- 
tion 



Spaulding. 
Tavlor 



ylor. 

R. nigrum "lacinia- 
tum": 

Sorauer 

R. odoratum: 

Gravatt 

Hedgcock 

Marshall 

Spaulding 

Taylor 

R. oxyacanthoides: 

Tubeuf 



Gravatt 

Lyman 

Merrill 

Spaulding 

Taylor.* 

R. parishii: 

Taylor 

R. petraeum: 

Gravatt , 

Spaulding 

R. petraeum atro- 
purpureum: 

Gravatt 

R. reclinatum: 

•Jaczewski 

Klebahn 

Sch^yen 

Sorauer 

Tubeuf'- 



Gravatt 

Hedgcock 

Spaulding 

R. reclinatum graft- 
ed on R. aureum: 

Klebahn 

R. reclinatum x 
missouriense: 

Gravatt 

R. reclinatum X ro- 
tundifohum: 

Gravatt 

R. robustum: 

Spaulding 

Taylor 

R.roezli: 

Gravatt 

Taylor 

R. rotundifolium: 

Sorauer 



Gravatt... 
Spaulding. 

Taylor 

R. rubrum: 
Jaczewski. 
Klebahn . . 



Rostrup . 
Sorauer. 



Num- 
ber. 



Susceptibility. 



In the 
green- 
house. 



XX 



XX 

XX 

XX 



Out of 
doors. 



XX 




Part seedling plants. 



20 BULLETIN 957, U. S. DEPARTMENT OF AGRICULTURE. 

Table I. — Results of inoculations made on various species and horticultural varieties of 

Ribes, etc. — Continued. 



Species or variety 
and investigators. 



Sec. 1. — Species of 
Ribes — Contd. 

R. rubrum "petrow- 
alskyanum": 

Spaulding 

Spaulding and 

Merrill 

R. rubrum pubes- 
cens: 

Spaulding 

R. rubrum scandi- 
cum: 

Merrill 

Rhoads 

Spaulding 

Taylor 

R. rubrum "sibiri- 
cum": 
Spaulding and 

MerrUl 

R. sanguineum: 

Neger , 

Sorauer 

Tubeuf 



Gravatt . 
St 



.aylor 
R. sanguineum 
"albidum": 

Gravatt 

R. sanguineum 
"floropleno": 

Gravatt 

Marshall 

R. setosum: 

Sorauer 



E 



Gravatt 

Spaulding 

ipeciosum: 

Gravatt 

Spaulding 

R. sterilis: 

Spaulding 

R. succirubrum: 

Gravatt 

Spaulding 

Taylor 

R. tenue: 

Gravatt 

Spaulding 

R. transbaicaulis: 

Spaulding and 

Merrill 

R. triste: 

Spaulding 

Taylor 

R. viburnifolium: 

Taylor 

R. villosum: 

Gravatt 

R. viscosissimum: 

Gravatt 

R. vulgare: 

Schc^yen 



Inocula- 
tions. 



Infec- 
tion. 



Num- 
ber. 



Hedgcock 

Spaulding 

R. vulgare macro- 
carpum: 

Spaulding 

R. wolfii: 

Spaulding 

*> Slight. 



Susceptibility 



In the 
green- 
house. 



XX 



XX 



XX 


XX 
XXX 

X 

> XX 

bX 

XX 



X 
XX 



Out of 
doors. 



XX 



XX 



Species or variety 
and investigators. 



Sec. 2. — Varieties of 
Ribes nigrum. 

Bang up: 

Gravatt 

Beauty: 

Spaulding , 

Black Dutch: 

Gravatt and Tay- 
lor 

"Black English": 
Colley, Gravatt 
Spaulding, and 

Taylor 

Black Grape: 

Spaulding 

Blacksmith: 

Gravatt 

Black Victoria: 

Gravatt, Mar- 
shall, and 

Spaulding 

Boskoop: 

Colley, Spauld- 
ing, and Tay- 
lor 

Carter: 

Gravatt 

" Cassis a fruit noir " : 

Gravatt 

Champion: 
Hedgcock, Mar- 
shall and 

Spamding 

Climax: 

Gravatt, Mar- 
shall, Merrill, 
and Spaulding. 
Clipper: 

Gravatt 

Collins: 

Spaulding 

Coronation: 

Gravatt and Taylor. 
Eagle: 

Gravatt and 

Spaulding 

Eclipse: 

Spaulding 

Edina: 

Gravatt 

Ethel: 

Spaulding and 

Taylor 

"French Black": 
Gravatt a n'd 

Spaulding 

Kerry: 

Gravatt 

Lee: 

Gravatt, Mar- 
shall, and 

Spaulding 

Magnus: 

Gravatt 

Mammoth: 

Gravatt 

Marvel: 

Gravatt, Spaul- 
ding, and Tay- 
lor 

Naples: 

Marshall and 

Taylor 

Norton: 

Gravatt 



Inocula- 
tions. 



Infec- 
tion. 



24 



Num- 
ber. 



Susceptibility. 



In the 
green- 
house. 



X 
XXX 

XXX 

XX 
X 
XX 



XX 
XX 
XX 



14 


XXX 


2 


XX 


1 


X 


3 


XX 


13 


XX 


2 


X 


4 


XXX 



X 




XX 


XX 


XX 


XX 


XX 


XXX 


XXX 


XXX 


XX 


XXX 


X 




XXX 


XXX 


X 





WHITE-PINE BLISTER RUST. 



21 



Table I. — Results of inoculations made on various species and horticultural varieties of 

fiibes, etc. — Continued. 



Species or variety 
and investigators. 



Inocula- 
tions. 



Infec- 
tion. 



Num- 
ber. 



Sec. 2.— Varieties of 
Eibes nigrum — Con. 

Ontario: 

Gravatt 

Saunders: 

Gravatt and 

Spaulding 

Seabrook: 

Spaulding 

Standard: 

Spaulding 

Success: 

Spaulding 

Topsy: 

Gravatt 

Wales: 

Spaulding 

Winona: 

Spaulding 

Woods: 

Gravatt 



Sec. 3. — Varieties of 
Ribes vulgarc. 

Admirable: 

Spaulding and 

Taylor 

Albert: 

Gravatt and 

Marshall , 

Angers: 

Merrill and 

Spaulding 

Berlin : 

Gravatt 

"Blanc de Werden": 

Gravatt 

Bonum: 

Gravatt 

Brandenburg: 

Spaulding 

Buddins: 

Gravatt 

"ChampagneWhite' ' 

Spaulding . . 

Chautauqua: 

Gravatt and 

Marshall 

Chenonceaux: 

Gravatt and 

Merrill 

Cherry: 

Marshall 

Comet: 

Marshall 

' < C ommun B lane " : 

Gravatt 

Conde: 

Spaulding 

Constant: 

Gravatt 

Crawford: 

Gravatt 

Cumberland: 

Gravatt and 

Spaulding 

Dilnot: 

Gravatt and 

Taylor 

Diploma: 

Gravatt 

Early Scarlet: 

Merrill and 
Spaulding 



Susceptibility. 



In the 
green- 
house. 



XX 

XX 

XXX 

X 

X 

XX 

XX 

X 

XXX 



XX 

X 

X 
X 
X 
X 

XX 
X 
X 

X 



Out of 
doors. 



XXX 



XX 



Species or variety 
and investigators. 



Sec. 3.— Varieties of 
Ribes vulgar e — Con. 

Everybodys: 

Gravatt 

"Eyath Nova": 
Merrill and 

Spaulding 

Fay: 

Gravatt 

Filler: 

Gravatt 

Fox Red: 

Gravatt 

Franco-German: 
Gravatt, Mar- 
shall and 

Spaulding 

Frauendorf: 

Gravatt 

"Giant Red": 

Gravatt and 

Marshall 

"Giant White": 

Gravatt 

Goliath: 

Spaulding and 

Taylor 

Greenfield: 

Spaulding and 

Tavlor 

Holland:" 

Gravatt 

Imperial: 

Gravatt 

' ' Improved Cherry ' ' : 

Gravatt 

Knight: 

Gravatt, Spaul- 
ding, and Tay- 
lor 

"Large Red": 

Spaulding and 

Merrill 

"Large White": 

Gravatt 

London: 

Gravatt and 

Marshall 

' ' Marvin Crystal" : 

Rhoads 

Moore Ruby: 

Gravatt, Hedg- 
cock, and Mar- 
shall 

New Red Dutch: 

Spaulding 

North Star: 

Colley and Tay- 
lor 

Norway: 

Gravatt 

Palluau: 

Gravatt, Spauld- 
ing and Taylor 
Perfection: 

Marshall 

Raby Castle: 

Spaulding 

Red Dutch: 

Spaulding and 

Tavlor 

"Red English": 

Spaulding 

Red Grape: 

Gravatt 



Inocula- 
tions. 



Infec- Num- 
tion. ber. 



Susceptibility. 



In the 
green- 
house. 



X 

X 
X 

XX 

X 
XX 

X 

X 
XXX 



Out of 
doors. 



XX 
X 
X 

bX 



*b X 



XX 



XX 
bX 






XX 


XX 


XX 


XX 


XX 


X 


X 


X 




X 


X 


X 




X 





I Slight. 



22 



BULLETIN 957, U. S. DEPARTMENT OF AGRICULTURE. 



Table I. — Results of inoculations made on various species and horticultural varieties of 

Ribes, -etc. — Continued. 



Species or variety 
and investigators. 


Inocula- 
tions. 


Susceptibility. | 

! 


Infec- 
tion. 


Num- 
ber. 


In the 
green- 
house. 


Out of 

doors. 


Sec. 3.— Varieties of 
Eibes vulgare — Con. 










Redpath: 

Spaulding 

Rivers: 



4 

1 

3 



2 

1 

10 

5 

4 

6 

6 
2 
1 

2 



3 
1 

4 

5 
11 

4 
1 

5 

1 

1 
2 
1 


1 
4 

5 
1 

o 

1 

10 

5 

o 

4 

7 



2 
1 

3 

1 

1 
3 
1 

4 

5 
11 

5 
1 

5 

1 

2 
2 
1 



id) 

X 

X 

X 

XX 
X 

XX 
X 

XX 

X 

X 
X 
X 

X 


X 
X 

X 

XX 
X 

XX 
X 






i 

1 

x ! 

X 


XX 

i 

---- 1 

"T 



X 

X 
X 

X 

X 
X 


Sablons: 

Spaulding and 
Taylor 


Scotch: 

Gravatt, Merrill 
and Spaulding. 
Simcoe King: 

Spaulding 

Skinner: 


Stria turn: 

Spaulding 

Transparent: 

Gravatt 


Turinoise: 

Gravatt and 
Taylor 


Utrecht: 

Gravatt 

Verrieres: 

Gravatt and 

Spaulding 

Versai liaise: 

Gravatt, Mar- 
sh all and 

Spaulding 

Victoria: 

Gravatt, Spauld- 
ing and Taylor 
Warner: 


Wentworth: 

Spaulding 

"W entworth 
White": 

Gravatt 


"White Bar le Due": 


"White Branden- 
burg": 

Spaulding 

White Champion: 

Gravatt 


White Cherry: 

Spaulding 

White Dutch: 

Gravatt and 

Marshall 

White Gondouin: 

Gravatt and 

Marshall 

White Grape: 

Marshall 


White Imperial: 
Gravatt, Hedg- 
cockand Tay- 


" White Kaiser": 
Spaulding 

' ' White Leviathan ' ' : 
Gravatt and 
Merrill 


White Pearl: 

Spaulding 

White Versailles: 

Merrill and 
Taylor 


Wilder: 

Spaulding 

Wilson: 

Marshall 




d Dead spots only. 











Species or variety 
and investigators. 



Sec. 4. — Varieties of 
Ribes reclinatum . 

Achilles: 

Spaulding 

Alma: 

R h o a d s and 

Taylor 

Berkeley: 

Gravatt and 

Marshall 

Carmen: 

Gravatt 

Carrie: 

Marshall and 

Taylor 

Champion: 

Gravatt and 

Taylor 

Chautauqua: 

Spaulding 

Columbus: 

Spaulding 

Cumberland: 

Gravatt and 

Spaulding 

Downing: 

Gravatt and 

Spaulding 

Duncan: 

Merrill and Tay- 
lor 

Golden Prolific: 

Gravatt 

Houghton: 

Gravatt and 

Spaulding 

Industry: 

Gravatt 

Josselyn: 

Spaulding 

Keepsake: 

Gravatt 

Lancashire Lad: 

Gravatt 

Mabel: 

Spaulding 

Mountain: 

Gravatt and 

Spaulding 

Oregon: 

Gravatt and 

Taylor 

Pearl: 

Gravatt and 

Taylor 

Poorman: 

Gravatt 

Portage: 

Gravatt 

Smith: 

Marshall and 

Spaulding 

Transparent: 

Gravatt and 

Marshall 

Triumph: 

Gravatt, Hedg- 
c o ck and 

Spaulding 

Van Fleet: 

Gravatt 

Victoria: 

Gravatt 

White Lion: 

Marshall 

Whitesmith: 

Gravatt 



Inocula- 
tion^. 



Infec- 
tion. 



Num- 
ber. 



Susceptibility. 



In the 
green- 
house. 





X 

XX 
X 

X 

X 

X 



X 
XX 

X 
X 

X 
X 




X 

X 

X 
XX 

x 

X 
X 

X 
X 
X 



WHITE-PINE BUSTER RUST. 23 

Recently Thayer (159) and Bunyard (8) have published results 
of their studies of the cultivated red and white currants. They find 
these currants to be of mixed and badly confused parentage, but con- 
clude that certain varieties sprang from each of the three species, 
Ribes vulgar e y R. rubrum, and R. petraeum. Many varieties are still 
to be assigned to the proper species; hence, they are grouped in Table I 
under the name R. vulgar 'e, for convenience, as it is yet impossible 
to assign all of them to any species. 

The gooseberries are well known to be in many cases a mixture 
of Ribes reclinatum with several American species or even pure selec- 
tions of American species. For convenience they are grouped under 
the species name, R. reclinatum. 

Susceptibility of Ribes Species and Varieties to Cronartium ribicola. 

Estimates of the susceptibility of the various species and varieties 
of Ribes have been made. (See Table I.) These are based on work 
done in the greenhouse and on results of the experiments out of doors 
on Block Island. These estimates have been made mostly by two 
persons, so that they are believed to be quite reliable and accurate 
by the standards chosen. The estimates for the inside experiments 
were made entirely independent of those out of doors. The two 
agree surprisingly. They are based on the results of work covering 
several years, but many of the species and varieties have been sub- 
jected to infection but a single year. A few species will be noted 
which have remained immune in our tests. But some of these 
species are known to have become infected elsewhere. This is true 
of Ribes alpinum which is reported to take this disease in Europe, 
although it is entirely resistant in North America (35, 53). Ribes 
innominatum has been well tested and took the disease only on newly, 
developed leaves. It is a very resistant species. Ribes sterilis, R. 
tenue and R. villosum have not yet undergone satisfactory tests, so 
that no conclusive statement concerning them can be made. 

The species of Ribes vary in susceptibility from the extremely 
susceptible Ribes nigrum to the very resistant R. leptanthum. On 
the former are produced the maximum number of uredinia and telia 
of the largest size, while on the latter the minimum number is pro- 
duced and these are poorly developed. Ribes alpinum has been 
entirely immune with us, although it takes the disease in Europe. 

The varieties of a cultivated species run fairly true to the species 
as a whole. Some real variation among varieties is believed to depend 
upon their mixed parentage. Many tests were necessarily made when 
the plants were not at the most favorable stage of development for 
the fungus to attack, and it is likely that further tests of aberrant 
varieties may bring most of them back into agreement with the species 
to which they belong. Of the varieties of the cultivated red currants 



24 BULLETIN 957, U. S. DEPARTMENT OF AGRICULTURE. 

which have been tested, the following are nearly immune: Eyath 
Nova, Franco-German, Holland (see also Tubeuf, 174), London, 
Rivers, Simcoe King. That is, plants tested under these names have 
so far shown themselves resistant, but not entirely immune. 

The cultivated gooseberries, varieties of Rites reclinatum in some 
cases more or less mixed with American species of gooseberries, are 
resistant but occasionally will become infected. 

Resistant species and varieties have been found reacting to the 
fungus and affecting it as follows : 

(1) Decreased number of uredinia and telia. 

(2) Above accompanied by reduction in size of uredinia and telia with lowered 

viability . 

(3) Small streaks and flecks of dead or dying tissue in infected leaves with 

uredinia and telia. (See PI. V, fig. 1.) 

(4) Small dead spots formed early with very few or no uredinia and telia. (See 

PI. V, figs. 3 and 4.) 

This agrees essentialty with the results of Stakman (149) with 
Puccinia graminis on resistant grains. 

LIFE HISTORY OF CRONARTIUM RIBICOLA. 
The Peridermium Stage on Pines. 

THE INCUBATION PERIOD ON PINES. 

According to European investigators, Cronartium ribicola has an 
indeterminate incubation period between infection of pine and pro- 
duction of aecia. This varies from about two to four years, and 
possibly much longer. In one of Klebahn's inoculation experi- 
ments he got pycnial drops on young white pines 11 months after 
the inoculation (71). In another instance, infection probably 
occurred in 1887, pycnia were produced in 1888, and secia in 1889 
(65). Recently Tubeuf (174) reported the results of successful 
inoculations on Pinus strobus. Inoculations were made on Septem- 
ber 11, 1914; pycnia formed in July, 1915; they were also produced 
in 1916 in May and thereafter; secia appeared in April, 1917. 

In North America considerable attention has been given to this 
matter. McCubbin (84) first attacked it by extensive studies of 
naturally infected trees. He concluded that five seasons were 
necessary for most of nearly 1,600 infections in Ontario to develop 
mature secia. He outlines the process as follows: first season, 
infection occurs; second season, dormant; third season, swelling of 
the bark; fourth season, swelling with pycnia; fifth season, mature 
aecia. This makes a lapse of about three years and six months 
between actual infection and the formation of mature sscia. 

Stone (153) in 1917 studied this problem in a locality where the 
fungus was fruiting on white pines for the first time after infection 
occurred. The infection came from Ribes cynosbati, which in 1914 
was heavily infected. The Ribes plants were removed in the spring 



WHITE-PINE BLISTER RUST. 25 

of 1915, so that infection was limited to the season of 1914. Of 40 
infections found, 12 were on 1913 wood and produced secia in May, 
1917; 28 were on 1914 wood and produced secia in May, 1917. The 
period of incubation for the former cases could not be more than 
three years and nine months. It may be that infection took place 
on 1 -year-old wood, in which case the incubation period would be 
only two years and ten months, as was the case with the 28 infections 
on 1914 wood. 

Study of infected branches which had just borne secia for the first 
time was made by Posey and Gravatt in 1917 at Stratham, N. H. 
Their notes show that more than 99 per cent of these infections 
might be 3 years and 6 months old, but could be no older. In 
another locality, they found a number of secia upon growth of the 
year 1915, making an incubation period of about 18 months. 

The first successful inoculations of pines with sporidia of Cronartium 
ribicola are apparently those made by Klebahn on August 27, 1903 
(71). Ribes nigrum leaves with telia were placed over two young 
trees of Pinus strobus and the whole covered with bell jars as long as 
the Ribes leaves remained fresh. On June 19, 1904, these trees had 
swollen twigs bearing juvenile leaves. Early in July, pycnial drops 
formed, after a lapse of 10 months. It is probable that in the normal 
course of events secia would form the next May. This would make 
an incubation period of about 19 months. ' 

The writer (133) in November, 1910, inoculated a number of 
healthy Pinus strobus trees in the greenhouse with teliospores. 
These inoculations were made on the young bark. In January, 1912, 
one each of those inoculated with wound and without wound of bark 
developed marked swelling. A little later pycnial drops formed, 
but snails ate them and the surrounding bark, so that the infections 
did not develop. Apparently it would have been a matter of but a 
few months when secia would have formed. This would give an 
incubation period of about two years. In May, 1916, the writer (145) 
set out healthy Pinus strobus trees among some experimental Ribes 
bushes on Block Island, R. I. ' The Ribes were heavily infected the 
rest of the season. Telia began to form the latter part of July and 
were abundant by September. On May 10, 1918, several of these 
trees were found bearing secia. This makes a maximum incubation 
period of about 21 months. 

In 1917, 10 young plants of Pinus flexilis were set out in the experi- 
mental plat on Block Island. Nine of them have lived. In the spring 
of 1920 seven of them bore secia of Cronartium ribicola on the growth 
of 1918, and the other two were much swollen and discolored, so that 
if alive they will certainly produce secia in 1921. It appears that P. 
flexilis is very susceptible. The experiments on Block Island indicate 
that it is even more susceptible than is P. strobus. The incubation 



26 BULLETIN 957, U. S. DEPARTMENT OF AGRICULTURE. 

period would be about 20 months. A number of P. strobus trees set 
in 1917 also bore secia in 1920, and many more will do so in 1921. 

Clinton and McCormick (12, 15) found that artificially infected 
trees of Pinus strobus, kept in the greenhouse, developed pycnial 
drops in five to six months after inoculation. 

All of the writer's experience in various outbreaks of this parasite 
shows that most of the newly formed secia are located on nodes and 
internodes that are 3 years old or over. It is rather exceptional for 
secia to be borne on needle-bearing wood 2 years old, in which case 
the minimum incubation period is about 18 months. The average 
incubation period out of doors is approximately 3 years and 6 months. 

TIME, PLACE, AND MANNER OF INFECTION OF PINES. 

There is constant danger of infection of pines at any time after 
telia form; that is, after about the 1st of June. The teliospores pro- 
duce sporidia in 6 hours under favorable conditions. 6 The sporidia 
germinate immediately. According to Clinton and Miss McCormick, 
infection of pine leaves may take place within 48 hours (14, 15) after 
the germinating sporidia are placed on the leaves. Any period of 
moist weather of 54 hours or longer after about June 1 may cause 
infection of pines. 

The available evidence indicates that infection of pine twigs takes 
place in or about the bases of the leaf fascicles (71, 84). If this is 
true for most cases, as seems likely, infection of Pinus strobus can 
occur only on wood that is 1 or 2, or exceptionally, 3 or 4 years old. 
This follows from the fact that the needles of this species ordinarily 
live only two seasons, but rather exceptionally they may live three 
or four seasons. 

Tubeuf (174) in 1917, published the results of successful inocula- 
tions with sporidia of Cronartium ribicola on pines. He inoculated 
Pinus strobus, P. lambertiana, P. excelsa, P. parviflora, P. pence, P \ 
cembroides, P. Jiexilis, P. montezumae, and P. cembra. He got 
yellow spots on the needles of P. lambertiana but no further results 
were noted. P. strobus became infected readily and bore secia, but 
none of the other species showed definite signs of infection. Infection 
evidently occurred in the needles, many of them having yellow spots. 
They were also present on the stems. Mycelium was abundant in 
the yellow areas. Tubeuf says that infection of the stem from the 
leaves is not common, but that direct infection of the stems is much 
more likely to occur. No pycnia were obtained on leaves, although 
the masses of mycelium in the yellow spots seemed to form the base 
for the pycnial spots in the bark. Older plants of Pinus strobus 
became infected less readily than those only 2 years old. This 

e York, H. H. Field studies of Cronartium ribieola in the White Mountains of New Hampshire. Seen 
in manuscript. To be published in Phytopathology. 



WHITE-PINE BLISTER RUST. 27 

infection of young plants he attributes to the fact that shoots bearing 
primary (juvenile) leaves go into the winter season with buds at all 
stages of growth, and many are incompletely protected by bud 
scales. Inoculations made on September 11, 1914, with sporidia 
succeeded on the primary leaves, on the secondary (mature) leaves, 
and on the epidermis of growing buds and of young shoots. Yellow 
spots were present on all these parts in the spring of 1915. 

Clinton and Miss McCormick (12, 14, 15) have recently announced 
successful inoculations in the leaves of Pinus strobus. Studies of 
thousands of infections show that infection takes place through the 
stomata of the pine leaves of all ages. Inside the stoma a sub- 
stomatal vesicle is formed which is of a characteristic shape. Thence 
the mycelium extends into the vascular bundle and then grows rapidly 
downward to the twig. Infection may take place in 48 hours. 
Inoculations on stems did not succeed. Inoculations on opened 
and unopened buds succeeded in a few cases, results being somewhat 
doubtful with the unopened ones. Infections on the leaves become 
visible, about a month after inoculation, as tiny yellowish spots 
centering on the line of stomata on the under side of the leaf. These 
spots turn golden yellow. Similar yellow spots may form on the 
twig after the fungus has become established there. A yellow 
mottling of the infected leaves is the principal symptom of the disease 
at this time. 

This is rarely noted in nature, but two instances having been seen 
by the writer before 1920 (131, 132, 133, 134). The spots were on 
both leaves and stems of naturally infected Pinus strobus trees. 
Kichards has recently found them on naturally infected leaves. 
So far as known they have been mentioned previously only by 
Tubeuf (174); Klebahn (71), who noted them on artificially infected 
trees but failed to designate the point of infection; and by Pechon 
(105), who observed them on naturally infected trees. The writer 
thought the yellow spots resulted from the growth of the fungus out- 
ward from the stem into the leaves. Klebahn seems to have been 
of the same opinion. It is believed that this happens sometimes. 
In 1920 such spots were seen on pine leaves naturally infected at 
Temple, N. H., and at New Boston, Mass. They are abundant in 
certain localities. Artificial infections have resulted from inocula- 
tions into the bark of the stem by the writer (133). 

Cronartium ribicola is able to grow in bark much more than 3 or 
4 years old if it once gains access to the interior. Infections have 
been seen which were still producing aecia on bark up to 35 years 
of age. A common method of infection of pine trunks 20 or more 
years of age is by growth of the mycelium from infected small side 
branches (135). (PL II, figs. 1 and 3.) Very often a twig near the 
base of a large branch becomes infected. The disease then extends 



28 BULLETIN 957 , U. S. DEPARTMENT OF AGRICULTURE. 

downward into the large branch, and from that into the main trunk 
of a tree, finally girdling it and killing the entire tree. This is true 
of nearly all of the older trees that have been killed in North America. 

TYPES OF INFECTION ON PINES. 

There may be said to be three types of infection on white pines 
resulting from natural inoculations. These are (1) direct infection 
of the main trunk on the leader; (2) direct infection of young branches 
or twigs; and (3) infection of an old trunk by spread of the mycelium 
from an infected branch. (PI. II, figs. 1 and 3.) All of these are 
present in outbreak areas in North America. Direct infection ap- 
parently occurs only on growth not more than 3 years~ old. Infec- 
tion of large branches or trunks, so far as we can judge, is limited 
only by the thickness of the bark. Old rough heavy bark of Pinus 
strobus was supposed to be immune to attack, but it has become in- 
fected by spread of the mycelium from infected side branches. It 
is a common method of entry of the fungus into older parts of a tree 
which were formerly supposed to be too old to become infected. 
It is very frequent in older outbreaks. This has not been mentioned 
in European literature until 1918 when Fischer (41) called attention 
to it. 

DIAGNOSIS OF BLISTER RUST IN PINE BARK BY MEANS OF THE MYCELIUM. 

In 1916, and to some extent before that date, when numerous 
specimens of diseased white pines were sent to the Office of Inves- 
tigations in Forest Pathology for quick and reliable diagnosis of the 
blister rust, many specimens were received which bore no fruiting 
bodies of the parasite. The appearance of many of these made it 
practically certain that they were infected with Cronartium ribicola. 
Colley (16, 19) studied the problem and shortly decided that the 
mycelium and the haustoria did furnish reliable evidence for 
identifying this parasite in the bark of Pinus strobus. The large 
intercellular hyphse, the large and abundant haustoria, and their 
manner of attacking the living cells, were found to be entirely differ- 
ent from the characters of any other known parasite of Pinus strobus. 
The use of these characters for four successive seasons with great 
numbers of specimens in various stages of development has indicated 
that such diagnosis of the disease is absolutely reliable. 

LONGEVITY OF THE MYCELIUM IN PINE SLASH. 

In November and December, 1916, some diseased native white 
pines were cut in outbreak areas in Ontario and in Maine, the slash 
being left lying upon the ground. Entirely independent observa- 
tions made by McCubbin in Ontario and by Posey in Maine early in 
May, 1917, showed that new secia were forming abundantly upon 



Bui. 957, U. S. Dept. of Agriculture. 



Plate II. 




;. * \ V^f| 




if * 


- 








S^; 


k/ 


'■? 




^ 


^i^-& 






*5 -rt*rfSH 


w' JErfH 


E^Stf^ 



.a 






£•* 






cfe/ 



Sections of Pine Trees, Showing Typical Progress of Blister- 
Rust Infection. 

Fig. 1.— Section of the trunk of a young tree of Pinus strobus, showing entrance of the 
blister rust into the trunk from an earlier infected branch. The dark shading on the 
trunk indicates the visibly infected portion. X 1. Drawn by J. M. Shull. Fig. 2. — 
Trunk of young tree of Pinus strobvs, about 2 inches in diameter, showing (by zona- 
tion) the progress of the blister rust. The disease entered the trunk from the side 
branch. The cracked bark (c) indicates where aecia have formed; the black spots (6) 
indicate the developing pycnia; the shaded part outside (a) indicates the area where 
the bark is discolored. X h Drawn by R. H. Colley. Fig. 3.— A trunk 8 to 10 
inches in diameter. The disease attacked the swollen twig first, then ran downward 
to the main branch, from which it has spread into the trunk. The latter will soon 
be girdled and the tree will die. Photographed by J. F. Collins. 



WHITE-PINE BLISTEK RUST. 29 

the cankers on this slash (89). This overwintering of the fungus 
by means of the mycelium is favored by the slash lying in moist 
places. It was also noted that a piece of a trunk several inches in 
diameter was producing new ascia after being kept in the dry air 
of the artificially heated laboratory in Washington about 6 weeks. 
These findings are significant, since they show that the cutting of 
diseased pines, if done in the late fall, winter, or early spring, must 
be accompanied by careful collection and burning of the infected 
slash, if infection of near-by Bibes is to be prevented. . This difficulty 
may be obviated by cutting the pines in summer when the following 
hot, dry weather will kill the slash and end the life of the mycelium 
within it. 

SEQUENCE OF PYCNIA AND JECIA AND PROGRESS OF THE DISEASE IN PINE BARK. 

Study of many blister-rust cankers of varying ages in the bark of 
large trunks of pine has shown that the disease extends through the 
bark in a regular and well-defined manner. Cankers of several years' 
standing usually consist of four distinct zones (20) (PI. II, fig. 2) : 

(1) An inner central zone of dead rough bark where secia have been borne in pre- 
ceding seasons. This area often has at its center a dead lateral branch or twig down 
which the disease has traveled from its first place of infection. This is the most com- 
mon method by which infection of large trunks and branches takes place. 

(2) An annual zone of living, swollen bark surrounding the dead area. Here is 
produced the latest crop of ascia. This zone varies in width from a fraction of an inch 
up to several inches. 

(3) A zone of discolored living bark bearing the pycnial spots, drops, or scars. 

(4) An outer zone of living bark, little or not at all swollen, showing a yellowing or 
bronzing of the normal green color of the smooth living white-pine bark. These zones 
of course move steadily outward with the progress of the mycelium. 

This sequence of zones of activity shows that we have a regular 
succession of events, as follows : 

(1) Invasion of healthy bark by the fungous mycelium, resulting in yellowing and 
bronzing of the invaded bark. 

(2) Formation of pycnia in discolored and often swollen, but still living bark. 

(3) Formation of secia in living bark which has previously borne pyncia. 

(4) Death of the bark which has borne secia in abundance. 

Investigations by Gravatt and Posey, and Colley (20, p. 650-651) 
indicate that the bark is often killed by invading secondary fungi as 
well as by Cronartium ribicola itself. Ehoads, 7 York, 8 and Pen- 
nington 9 found that occasional patches of bark, where aecia have 
been borne the previous year, remain alive and bear a second crop 
of secia the second season. 

i Rhoads, A. S. Studies on the rate of growth and behavior of the blister rust on white pine in 1918. 
Seen in manuscript. Published in Phytopathology, v. 10, p. 513-527. 1920. 

8 York, H. H. Op. cit. 

9 Pennington, L. H. Investigations on the white-pine blister rust in New York. Seen in manuscript. 
To be published as Tech. Bui.. N. Y. State Col. Forestry. 



30 BULLETIN 957, U. S. DEPARTMENT OE AGRICULTURE. 

Rhoads 10 came to the conclusion that the disease spreads from the 
original point of infection upward and downward at about equal 
rates of progress. Posey and Gravatt, and Rhoads found that it 
spreads at nearly the same rate laterally on both sides. Progress 
upward and downward is usually more rapid than it is sidewise. It 
also appeared that in infections not yet bearing secia, the point of 
greatest swelling is where infection first took place and where aecia 
will be first produced. Very often a dead twig or a leaf scar shows 
very plainly the original point of entrance of the fungus to the bark. 
Rhoads concluded that infections on shaded lower branches. do not 
spread as rapidly as those on vigorously growing ones, but Posey and 
Gravatt n find that the former are more likely to be attacked by 
secondary fungi, which soon kill- the branches. 

Posey and Gravatt n found that there are more or less frequent 
instances in old infected areas of white pines where the infections on 
lateral branches die out. The statement (131, p. 16; 141, p. 5) that 
trees once infected with this fungus never recover was largely based 
upon studies of trunk infections. Like all rules, it has its exceptions, 
as here indicated. At Kittery Point, Me., Posey and Gravatt 
studied one of the oldest outbreaks in North America. Trees of all 
ages from a few years up to 50 or 75 were infected. Here it was found 
that secondary fungi often kill the blister rust in an infected branch 
and that increasing suppression of lower branches killed many of the 
infected ones before the blister rust spread to the trunk of the tree. 
It was found that about 15 per cent of all the infected trees in the area 
studied recovered from the disease by the action of these two factors. 

THE PYCNIA AND PYCNOSPORES OF CRONARTIUM RIBICOLA. 

The pycnospores of the Uredinales have received comparatively 
little attention, since it has been generally accepted that they are 
apparently functionless (10, 21, 70, 110, 122, 155). The writer can 
find but little data upon which this idea is based. Plowright (110), 
Thaxter (158), Jaczewski (47), and Klebahn (68) are the only investi- 
gators known to the writer who have actually inoculated plants with 
the pycnospores of their rusts. It seems that the pycnospores should 
be more thoroughly tested. 

The work with pycnospores of Cronartium ribicola in Europe seems 
to be limited to that of Klebahn (68), who made inoculations with them 
upon young Pinus strobus trees without infection occurring. Even 
in this case there was not a clear-cut result, such as is to be desired. 
More recently Colley (18) has shown the importance of the pycnial 
spots, drops, and scars as symptoms of the blister rust in pine bark, 
and still more recently (20) he has investigated their morphology and 

cytology. 

. • . , 

io Rhoads, A. S. Op. cit. 

11 Posey, G. B., and Gravatt, G. F. Field studies on the white-pine blister rust at Kittery Point, Me. 
Seen in manuscript. 



WHITE-PINE BLISTER RUST. 31 



GERMINATION OF THE PYCNOSPORES. 

Plowright states that pycnospores (spermatia) of some of the 
Uredinales have been germinated in sugar solutions by Cornu and 
himself (110). Brefeld (7) states that the spermatia of several rusts 
have been germinated in culture solutions. Later Carleton (10) 
stated that he had been able to germinate the pycnospores (sperma- 
tia) in but a single instance. 

The spermatia of Uredo caeoma-nitens Schwein., budded sparingly on May 31, 1893, 
after 24 hours in a dilute solution of honey, but would not germinate in water. 

Still later he said (11): 

Until recent years it was not supposed that the spermatia produced regular germ 
tubes, but that the germination is always simply a process of budding. Dr. N. A. 
Cobb and the writer have shown, however, that ordinary germ tubes are produced in 
the germination of these spores as well as in the other spore forms * * *. Spermatia, 
though germinating readily in water, will be found to do much better in a rather 
dilute sugar solution, or perhaps still better in a solution of honey. 

Investigations were made in 1918 by York and Overholts, 12 who 
tested their germination in water and in various solutions of glucose, 
cane sugar, dextrose, maltose, lactose, peptone, extract of macerated 
Kibes leaves, and extract of macerated pine bark. Fresh pycnospores 
gave no germination. Pycnospores subjected to the cold of an ordi- 
nary refrigerator from 3 to 20 days gave degrees of germination 
increasing with the length of time in cold up to 18 days. Germina- 
tion occurred in 3, 5, and 6 per cent cane sugar and in 3, 6, and 10 
per cent dextrose. The stronger dextrose solutions gave the best 
results. No germination occurred in tap water in any case. 

SEASON OF PRODUCTION OF PYCNIAL SPOTS, DROPS. AND SCARS. 

The dark spots (PL II, fig. 2, b) which precede the exudation of the 
pycnial drops may be found in the infected bark at all times of the 
year. Records and observations made from 1909 to date furnish 
definite data on the season of pycnial drop formation. (See Table V, 
p. 72.) The pycnial drops are produced immediately after the 
fecial season; that is, from about June 20 until winter. The rusty- 
brown scars left after the disappearance of the drops may be seen 
at all times of the year on old cankers. On new infections the scars, 
of course, do not appear until the drops have formed and disappeared. 
All of these are positive symptoms of this disease in white pines. 

THE ^CIA AND ^CIOSPORES OF CRONARTIUM RIBICOLA. 

SEASON OF PRODUCTION OF THE ^ECIA. 

The secia develop at varying dates in the same locality in different 
years, according to the season. The locality, whether a warm or 
cold exposure, at a low elevation or a high one, well to the south or. 

12 York, H. H., and Overholts, L. O. The germination of the pycnospores of Cronartium ribicola. Seen 
in manuscript. To be published in Phytopathology. 



32 BULLETIN 

far north, has much to do with the time that the secia appear. They 
begin to push through the bark several weeks before they break open 
and distribute the seciospores. Open blisters have been noted as 
early as April 5 in eastern Massachusetts, but they were nearly three 
weeks later in the Adirondacks the same spring. Table V (p. 72) gives 
specific data so far as they are available for this and related dates. 

Klebahn (68) stated that seciospores are produced by an secium of 
Cronartium ribicola for more than 14 days. Posey and Gravatt 13 
found that seciospore production took place at Kittery Point, Me., in 
1917, from April 29 to July 1. 

In the spring of 1918, Rhoads made observations at Kittery Point, 
Me., to determine how long individual secia produce spores. 14 On 
May 3, he stuck pins, bearing numbered piec&s of paper, into 300 
secia on various trees just as they first broke open. On May 20, the 
peridia of all but 63 of the secia were entirely gone with most of the 
spores. On May 29 remnants of but 19 were left and on June 4, 
none. Therefore, in 1918 the seciospore season was about 4 weeks long 
at Kittery Point, Me. 

York 15 working at North Conway, N. H., in 1918, found that a few 
individual secia contain viable spores for from 20 to about 30 days. A 
study of secial production on single cankers showed that secia matured 
for a period of about 30 days. These infections were all on relatively 
small twigs and branches. On larger branches or trunks the period 
may be longer. Study of the period of secial production in that entire 
region showed that it was approximately 70 days. At Lewis, N. Y., 16 
the secial season in 1919 was slightly more than two months in length. 
After the main secial season, late straggling secia form. Rhoads 14 
noted that secia occasionally develop on areas of bark which bore 
secia the preceding year, but which were still alive. 

York noted a newly formed secium of Cronartium ribicola near 
Littleton, N. H., on July 21, 1918 (179). Several still more remark- 
able cases were noted by York and Ninman at Amery, Wis., on 
September 15 and 16, 1919. Such late secia appear to be unknown 
for any of the other stem Peridermiums except C. occidentals, which 
has an secial season from June to August. Numerous instances are 
known where C. ribicola was entirely absent on Ribes in a given 
locality early in the season, but later was found to be present in greater 
or less abundance. Some of these infections may originate from 
such late secia. These late seciospores may remain viable over winter 
in the secia, since Dosdall (29) has found that seciospores occasionally 
retain viability until the next spring. 

13 Posey, G. B., and Gravatt, G. F. Op. cit. w York, H. H. Op. cit. 

14 Rhoads, A. S. Op. cit. 1 « Pennington, L. H. Op. cit. 



WHITE-PINE BLISTER RUST. 33 

Observations by Pennington 17 on the aecia showed that more secia 
were produced per canker and many new cankers were fruiting in 
1919, so that more seciospores were produced that year than in 
1918 in the Adirondacks. Observations on the first generation of 
uredinia and the results of spore- trap work indicated, however, that 
not as many seciospores were set free in 1919 as in 1918. This is 
supposed to have been due to heavy rains, which beat them down 
from the air. The distribution of the first generation of uredinia 
showed that the seciospores were as widely disseminated in 1919 as 
in 1918. 

DISTANCE OF DISSEMINATION OF THE .ECIOSPORES. 

As is true of many of the more difficult points in the life history of 
Cronartium ribicola, European statements concerning the distance 
that the seciospores are distributed are based apparently mostly on 
personal opinions. The value to be attached to these statements 
seems to rest on the known excellence, or the reverse, of the pub- 
lished work and judgment of the writer who is being considered. 
European mycologists have mentioned a number of instances where 
this fungus appeared on Ribes which were said to be far removed from 
white pines. But some of these cases were later found to be actually 
much nearer diseased white pines than was at first supposed. Tubeuf 
(166) has stated that the seciospores spread the disease up to 500 
meters or more. On the other hand, a considerable number of 
definite recommendations for separation of the alternate hosts set a 
distance of only 30 to 100 meters (131, p. 41-42). 

In North America earlier field experience indicated that the 
seciospores spread the disease for rather short distances from their 
source. It was recognized from the beginning that these spores are 
exceedingly light and well adapted to wind dispersal, and it was stated 
that our knowledge of their dispersal was very limited. Special 
efforts have been made for three seasons to gather more data on this 
problem. 

Posey, in 1917, set bushes of Ribes nigrum in a salt marsh at Kittery 
Point, Me., at varying distances from infected pines before uredinia 
were formed. Some infection resulted from seciospores upon Ribes 
plants more than a quarter of a mile from any white-pine trees. 
The infecting spores probably traversed several hundred yards addi- 
tional, as the nearest pines were not known to be diseased. The 
heavily infected area was about 1J miles distant, and the infecting 
spores may easily have come that distance. 

Posey also examined the islands of the Isles of Shoals, off Ports- 
mouth, N. H., for the presence of Cronartium ribicola . He found no 
pines, but a number of Ribes hirtellum plants. A few leaves were 

» Pennington, L. II. Op. cit. 

46103°— 21— Bull. 957 3 



34 BULLETIN- 957, U. S. DEPARTMENT OF AGRICULTURE. 

found infected with the fungus. In 1920, Snell reexamined these 
islands much more carefully. He again found the fungus on several 
leaves and decided that it is evidently a case where the seciospores 
were blown from infected pines to the islands. The islands are about 
7 miles from the mainland, so that it appears that the seciospores may 
be blown this distance and infect Ribes. There appeared to be no 
reason for thinking the fungus wintered over. At any rate, it must 
have come from the mainland originally. The islands where the 
disease was found are very seldom visited, so carriage of spores in 
this way appears to be eliminated. 

McCubbin (88) found that the seciospores fall about 8 feet in seven 
minutes in still air. This indicates a very wide potential distribution 
of these spores by a moderate breeze. 

In 1918, York and Overholts (cited in Spaulding, 145) worked in 
the White Mountains of New Hampshire in a generally infected 
region. Much work was done with spore traps and much time spent 
in examination of Ribes plants which were isolated from white pines. 
The work proved that the seciospores are distributed for miles to the 
tops of adjacent mountains approximately 3,000. feet high, that they 
arrive in a viable condition, and that they are the means by which 
the disease spreads far and wide to Ribes. 

In 1918, also, Pennington and Snell (cited in Spaulding, 145) 
worked in the eastern Adirondack region of New York. Spore traps 
here gave valuable contributory evidence, but study of the distribu- 
tion of the first generation of urediniospores with reference to neigh- 
boring white pines gave the best results. Here it was found that 
spore traps caught seciospores up to 550 feet from any pines. Within 
a large area of cultivated land at Essex, N. Y., an intensive study was 
made by Snell (128) of the first generation of urediniospores. In this 
area the Ribes were found to have first-generation uredinia sparingly 
and widely scattered; that is, the seciospores causing the infection 
evidently came from a considerable distance. In one case diseased 
Ribes were found three-fourths of a mile from any white pine. 
Several others were found at smaller distances from any pine trees. 
It was concluded that the seciospores came from a distance of not less 
than three-fourths of a mile and probably much farther. 

These conclusions concerning the wide spread of seciospores in the 
two localities were arrived at independently and without the knowl- 
edge by either party of what conclusions had been reached by the other. 

In 1919, Snell (128) found near Rush Lake, Minn., infections on 
Ribes leaves which were 1J^ miles from the nearest pine and about 
3 miles from the nearest known diseased pine. Many such infections 
were found in the same general area which were half a mile or more 
from any pine. These infections were found developing when the 
first generation of uredinia appeared throughout that general in- 



WHITE-PINE BLISTER RUST. 35 

fected area. They must have been produced by aeciospores which 
had been blown at least the above distances. 

In 1919, York caught and germinated aeciospores on the summits 
of two mountains nearly 4,300 feet above the adjoining lowlands. 
It is evident that altitudes such as this do not prevent the spread 
of this fungus. Pennington 18 caught aeciospores up to 1,200 feet 
distant from pines and found diseased Ribes three-fourths of a mile 
from any pine tree. 

AGENTS DISSEMINATING THE ^CIOSPORES. 

It has been evident from the beginning that wind is a most efficient 
and active agent in the distribution of the spores of Cronartium 
ribicola. While the probability of spore carriage by other agents 
such as insects and the larger animals was recognized, no time could 
be spared for work upon it. More recently, Gravatt and Marshall 
(45) and Gravatt and Posey (46) have made some studies of this 
sort. 

Gravatt and Marshall worked in the experimental greenhouse 
where no aecia were present. They found that pycnia and the 
surrounding bark tissues were eaten by sow bugs. 

Gravatt and Posey (46) made studies in the field in a heavily 
infected pine area at Kittery Point, Me. Here it was found that 
gipsy-moth larvae, which were abundant, fed eagerly on the 
pycnia and aecia of the blister rust and also ate the bark tissues 
immediately adjacent to them. It was found that in some cankers 
a considerable percentage of the fruiting aecia were thus destroyed. 
But a few ingested spores remained viable, as tests in hanging drop 
cells in tap water yielded a few germinations. These larvae also 
were carriers of abundant aeciospores on their bodies, many being of a 
decided yellow color from the spores with which they were dusted. 
The gipsy-moth larvae are known to have been blown 20 miles or 
more. Within the gipsy-moth infested area these larvae are then 
dangerous agents in the distant spread of the disease, a fact not 
previously appreciated. 

Notes made by Gravatt show that a wood mouse caught in the out- 
break area at Kittery Point, Me., carried a small number of aeciospores 
on its body. While squirrels, other animals, and birds have not been 
examined, there can be no doubt that they are active carriers of the 
spores. It is known that the aeciospores become attached readily 
to clothing, and there can be no doubt that persons may carry the 
disease, at least locally, in this manner. 

In a number of outbreak areas where pine infections were just about 
to produce pycnia for the first time, it was noted by several observers 19 

18 Pennington, L. H. Op. cit. 

1 9 Rhoads, A. S. Op. cit. 



36 BULLETIN 957, U. S. DEPARTMENT OF AGRICULTURE. 

(143, 144) that squirrels ate the swollen bark from the infected parts 
of the branches. (PL III.) They undoubtedly run over fruiting 
cankers and pick up aeciospores on their fur and feet. Porcupines 
may act in the same manner. Birds undoubtedly carry these spores 
to some extent, and as in the aecial season they begin nesting and 
largely remain in the same locality they, too, act only as local carriers. 
Some indications have been noted where a road traverses a narrow 
valley, or a narrow clearing in a forest, that automobiles create drafts 
which carry spores to some distance along the highway. It seems 
entirely possible for steam trains and electric cars to do the same 
thing. 

POSSIBLE AUTCECISM OF THE ^EGIO SPORES. 

The possible autcecism of the aeciospores of Cronartium ribicola has 
been considered. As early as 1913 field observations were made with 
this point in view, but no evidence of the spread of the fungus directly 
from pine to pine was found. 

The question whether aeciospores of other stem-inhabiting pine 
Peridermiums are capable of infecting pines has received considerable 
attention. In 1907 Liro (82) stated that he had made 169 inocula- 
tions of Finns sylvestris with aeciospores of Peridermium pini from 
the same host. No infections resulted. In 1914 Haack (48) stated 
that fee had made similar inoculations and obtained abundant infec- 
tions. His experiments were performed out of doors, with no pro- 
tection from natural infection and with trees which already were 
naturally infected ; hence, his results are worthless. In 1913 Meinecke 

(95) made successful inoculations with aeciospores of "Peridermium 
Ttarknessii" upon Pinus radiata under controlled conditions. Later 

(96) he changed the name of the fungus to Peridermium cerebrum and 
reported that he had successfully inoculated Pinus radiata with 
aeciospores from P. radiata and from P. attenuata; and P. muricata 
with aeciospores of P. cerebrum from P. radiata. 

In 1915 Hedgcock (51) successfully inoculated trees of Pinus 
ponderosa var. scopulorum, P. contorta, P. sabiniana, P. caribaea, 
P. mariana, P. pinea, and P. Jialepensis with aeciospores of u Peri- 
dermium liarknessii" from P. contorta. He has also successfully 
inoculated P. ponderosa and P. virginiana with aeciospores of "P. 
liarknessiV 1 from P. ponderosa. 

In 1918 Klebahn (73) published the results of successful inocu- 
lations made by him with aeciospores of Peridermium pini upon 
young twigs of Pinus sylvestris, both with and without wounds, 
under controlled conditions. These results throw doubt on the 
strict heteroecism of the aeciospores of all the stem-inhabiting pine 
Peridermiums. 

The following tests have been made with the aeciospores of Cro- 
nartium ribicola: Klebahn (68, 70) repeatedly inoculated young 



Bui. 957, U. S. Dept. of Agriculture. 



PLATE III 




Trunk and Branches of Pinus strobus, Showing Bark Infections of 
Blister Rust Eaten by Squirrels. 

Photographed by W. H. Snell. 

Fig. 1.— An infected branch which was evidently a young infection that had not yet formed 
aecia. Fig. 2.— Infected bark of living tree. Here may be seen the blister-bearing central 
area at the base of the branches in place, while the outer, surrounding, pycnial zone has 
been eaten away. Fig. 3.— Infected bark of living tree. The eaten parts were where pycnia 
were forming. 



Bui. 957, U. S. Dept. of Agriculture. 



PLATE IV. 





/*\A% 





Leaves of Ribes Infected With Cronartium ribicola, Showing Different 

Types of Attack. 

Fig. 1.— Lower surface of a leaf of Ribes aureum infected by Cronartium ribicola. Note the charac- 
teristic isolated infected areas, with the abundant uredinia closely crowded together. Fig. 2. — A 
Ribes bud with a single leaf which bears normal uredinia. This leaf is relatively old, being 
stunted in growth by adverse conditions which have held it stationary for several weeks. Fig. 3 — 
Lower surface of an infected leaf of Ribes vulgare, horticultural variety White Transparent. Note 
the large infected areas merging into a single one. The uredinia are not so closely crowded 
together as in figure 1. Fig. 4. — Lower surface of an infected leaf of Ribes nigrum. Note the 
general distribution of the telia, their grouping closely together, and their vigor of growth. 



WHITE-PINE BLISTER BUST. 37 

Pinus strobus trees with seciospores, but without producing infection. 
Hennings (53) inoculated P. strobus trees with seciospores and also 
with teliospores. No infection resulted from either. In the spring 
of 1916 the writer made 100 inoculations with wounds into the bark 
of Pinus strobus trees, out of doors, with fresh seciospores of Cronar- 
tium ribicola. No infections have resulted to date. In May, 1917, 
the writer (146) inoculated white pines by dipping the tips of young 
twigs in water containing great quantities of newly formed seciospores 
of C. ribicola. The needles as well as the twigs were covered with 
spores. Glassine bags, containing wet wads of cotton, were then 
tied over the inoculated twigs to keep up the humidity of the air. 
No evidence of infection is yet visible. 

GERMINATION OF THE yECIOSPORES. 

Experience shows that fresh seciospores taken from secia just as 
they are about to break open, or just at the time of breaking, possess 
maximum infective power. Doran (28) confirms this opinion. 
Inoculations made with such spores are sure of results if conditions 
are at all favorable. Older seciospores are erratic in germination, 
but some of them retain viability to a marked degree. Cooling on 
ice stimulates their germination to a decided degree, as is shown by 
experiments performed by Eriksson (32), Gravatt, and others. 
Each spore produces from one to five (20, 29) germ tubes, which 
branch freely. The viability of fresh seciospores is generally high, 
as many as 95 per cent germinating under favorable conditions. 
They require 8 to 10 hours to germinate (28). 

Doran (28) determined the minimum, optimum, and maximum 
temperatures for the seciospores of Cronartium ribicola. Five series 
of tests were made. It was found that germination in distilled-water 
drop cultures occurred through a range of 12° C, but the percentage 
of germination dropped rapidly both above and below the optimum. 
The minimum temperature for the seciospores was 5° C, the optimum 
was 12° C, and the maximum was 19° C. 

LONGEVITY OF THE ZOOSPORES. 

Klebahn (70, p. 26) seems to have been the only European investi- 
gator who has tested the longevity of seciospores. He found them 
strongly viable after seven weeks. 

From the beginning of the work of the writer on Cronartium 
ribicola it has been known that the seciospores retain their viability 
for a relatively long time under favorable conditions. In 1910 a 
single seciospore which had been kept in the laboratory for more 
than five months in an open secium on a diseased young tree which 
was dried and kept as a specimen germinated in water (131, p. 30). 
In 1915 McCubbin (85) collected on May 6 a diseased young white 



38 



BULLETIN 957, U. S. DEPARTMENT OF AGRICULTURE. 



pine bearing ascia. It was placed in a box in the laboratory and 
allowed to dry out. Tests were made by inoculating leaves of Ribes 
nigrum plants on .May 7, May 21, June 4, and June 23. On the last 
date no infection occurred. In 1916, McCubbin repeated these 
experiments and found that the seciospores remained capable of 
infection under the conditions of his experiment at least 39 days. 
He believes they may retain viability considerably longer, as his 
tests were conducted under adverse conditions. 

iEciospores collected on April 9, 1917, and kept in a closed glass 
vial were tested by Gravatt weekly in hanging drops of distilled water. 
They gave good germination for about one month, then weakened 
until June 9, when the last germination occurred. The tests con- 
tinued until July 14. On May 7, 1917, another series of similar tests 
was started by Gravatt and Taylor. (See Table II.) The asciospores 
were placed in two glass vials with cheesecloth tied over the tops. 
One vial (B) was kept out of doors on a window sill on the north 
side of a building where the sun did not shine. The other vial (A) 
was kept in a dark refrigerator. The spores were tested weekly in 
distilled water. In most cases the cultures were placed in an ice 
box for about 12 hours. The spores kept in the ice box retained 
their color throughout, while those on the window sill had faded 
perceptibly by June 16. Lot A varied in germination from 8 per 
cent at first to 3 per cent on June 2. Germination persisted until 
July 2, when the last occurred in this lot. Lot B germinated freely 
until May 26 and not at all after June 23. Cooling and darkness in 
this case decidedly stimulated germination for one month and pro- 
longed viability about 10 days after the uncooled asciospores which 
were exposed to light had lost all viability. 

Table II. — Longevity of the spores of Cronartium ribicola. 





Germination (per cent). 




Date, 
1917. 


Lot A, vial in 
refrigerator. 


Lot B, vial out of 
doors. 


Notes. 




Mcio- 
spores. 


Uredi- 

nio- 
spores. 


Telio- 

spores. 


vEcio- 
spores. 


Uredi- 

nio- 
spores. 


Telio- 

spores. 




May 8. 
May 12. 
May 19. 
May 26. 
June 2. 
June 9. 
June 16. 
June 23. 
July 2. 
July 7. 
July 14. 


8 
8 
5 
4 
3 
1 

.1 


.05 




13 
10 
60 
45 

6 
.65 

1.35 










90 
85 
80 
70 
40 
51.3 

4.5 

3.5 
.5 






8 
1 
2 
3 

.8 

.1 


.05 





8 

10 

40 

37 

.8 

1.5 




.1 






85 
90 
85 
50 
10 

4.4 






Not cooled in refrigerator before germination . 
Do. 

Lot A somewhat faded. 
Lot A much faded. 
Teliospores in lot A mouldy. 

JEciospores and urediniospores mouldy in 
lot A, color of lot B still fairly good. 



WHITE-PINE BLISTER RUST. 39 

In April, 1918, Dosdall tested in distilled water (in hanging-drop 
cultures) the viability of seciospores produced in 1917 (29). On 
April 19, 1918, a dead white-pine branch bearing a canker which 
had fruited in 1917 was collected at Rush Lake, Minn. While new 
secia were just beginning to break open on other cankers at this 
time, the spores tested were not new ones, as they were dug from the 
bottoms of 1917 cavities after scraping off the outer exposed spores. 
Nor could there have been new secia pushing up beneath the old 
ones, as the branch was dead. It was found that from 1 to 2 per 
cent of the spores germinated in distilled water, each spore producing 
from 3 to 5 germ tubes. It is barely possible that these spores 
were from an abnormally late a3cium (179) and therefore were not so 
old as Dosdall supposed them to be. Even so, they must have been 
approximately 6 months old. 

This experiment of Dosdall has been repeated. A dead branch 
bearing secia of 1918 was collected at Kittery Point, Me., on Febru- 
ary 25, 1919, and taken to Washington, D. C. Taylor tested the 
seciospores by hanging-drop cultures in tap water, but no germina- 
tion of the spores could be demonstrated. Spores of other fungi 
were present and did germinate. 

York 20 collected a specimen of diseased white pine bearing newly 
formed secia on April 30, 1918. This was put in a paper bag and left 
in the laboratory away from direct sunlight until October 5, 1918. 
He then broke open a still unbroken secium which had not pushed 
through the outer bark and made cultures of the spores. He got 
some germination in tap water under these condition 157 days after 
collection of the material. The spores were still yellow when the 
test was made. Spores from secia which had broken open did not 
germinate. 

During the season of 1918, Pennington 21 found that in the Adiron- 
dacks the seciospores remained viable for at least four weeks after 
being removed from the secia and stored in a dry place. The same 
season, York found that in the White Mountains seciospores from 
blisters in cankers cut from the tree and kept in the shade out of 
doors remained viable for 75 days, as shown by tap-water cultures 
and inoculations on Ribes leaves. 

In 1919, Pennington 21 found that seciospores, whether brought 
into the laboratory or left in the field soon lost their viability, less 
than 1 in 400 germinating after three weeks from the breaking open 
of the secium producing them. Tests upon the viability of seciospores 
after they had been exposed to direct sunlight showed a decrease of 
50 to 75 per cent in viability after three hours' exposure. After 
an exposure of eight hours, some of the seciospores (1 in 1,500 or 
2,000) were still viable. 

20 York, H. H. Op. cit. 

21 Pennington, L. H. Op. cit. 



40 BULLETIN 957, U. S. DEPARTMENT OF AGRICULTURE. 

Dor an (28), in 1919, found that the seciospores germinate well in 
distilled-water drop cultures for four weeks, when germination 
weakens. Germination ceased after six weeks. He does not give 
details of the conditions of storage of the spores. 

Pennington 22 compared the number of aeciospores caught in spore 
traps with the number of infections on Ribes leaves and estimated 
that under the most favorable conditions 1 spore in every 25 which 
lodged upon a Ribes leaf produced infection there. An estimate 
for the season showed that not more than 1 in 100 produced infection. 

The Cronartium Stage on Ribes. 

THE INCUBATION PERIOD ON RIBES. 

The length of the incubation period between infection of Ribes 
by aeciospores and urediniospores of Cronartium ribicola and the 
production of mature uredinia or telia varies greatly, according to 
the external conditions of temperature and moisture and the age 
and condition of the leaves infected. Examination of the records 
of 493 inoculation tests made in the greenhouse show that the shortest 
incubation period between infection and formation of mature uredinia 
is practically 7 days. These records show that 2.4 per cent of the 
cases fruited in 7 days, a like number in 8 days, 7.3 per cent in 9 
days, 10.4 per cent in 10 days, 20.8 per cent in 11 days, and 19.7 
per cent in 12 days. The percentage rapidly drops after this to 9.7 
per cent in 13 days, 8.1 per cent in 14 days, 8.5 per cent in 15 days, 
8.1 per cent in 16 days, and 3.2 per cent in 17 days. 

Pennington 22 found that the incubation period on Ribes in the 
eastern Adirondack region with both seciospores and urediniospores 
was 11 to 18 days; it was usually 13 to 15 in mature leaves and some- 
what longer in leaves which were very young when inoculated. 

There are times when the fungus produces only uredinia in the 
greenhouse as well as in the fields. The behavior of the fungus is 
not entirely controlled by weather conditions, but is greatly influ- 
enced by the condition of the host leaves. At other times the 
fungus will produce nothing but telia. At such times telia are pro- 
duced in a very short time after infection. Telia have been obtained 
in 9 or 10 days after infection. From 12 days upward they may be 
formed at almost any time up to 2 or 3 months after infection. 

York 23 in many cases has obtained telia directly from aeciospore as 
well as urediniospore inoculations upon overmature 24 leaves. 

22 Pennington, L. H. Op. cit. 

23 York, H. H. Op. cit. 

2* The term "overmature" is here used to denote that stage of development of Ribes leaves where they 
have become tough, leathery in texture, and of maximum thickness, but have not begun to decline in 
photosynthetic activity. 



WHITE-PINE BLISTER RUST. 4l 

METHODS OF INOCULATING KIBES. 

The methods of inoculating Ribes plants are not claimed to be 
original with the writer or his associates. It is well known that 
some of these methods have been in use for many years. They are 
given here to show the conditions under which the experimental 
work was done,, as follows: 

(1) When plenty of spore material is available, as is usually the case with secio- 
spores, the spores may be placed in a considerable quantity of water and the top or 
branch of the Ribes plant dipped into it. The spores will be distributed quite 
evenly over all parts of the dipped plant. (See Clinton (12).) This method was used 
by the writer as early as 1909, and has been very successful. It uses up large 
quantities of inoculum, however. 

(2) Another method which has been much used is to spray water from an atomizer 
upon the part to be inoculated, then .shake the dry spores upon the wet surface. 
This also requires a plentiful supply of inoculum. It has been a favorite method, 
as it gives good results with reasonable certainty. 

(3) When inoculating with urediniospores from fresh leaves, the part to be inocu- 
lated is sprayed, and the leaf bearing the inoculum is turned with its lower surface 
on that of the healthy leaf and the two rubbed lightly together. 

(4) If the inoculum is scanty, the spores are moistened with a drop of water and 
lightly scraped off upon the moistened healthy leaf with a sterilized scalpel or knife 
blade. 

(5) If the inoculum is in moderate quantity, the spores are placed in a small 
quantity of water in a sterilized atomizer and sprayed upon the healthy plant. 

(6) The spores, if fairly plentiful, are sometimes collected in a watch glass, a small 
quantity of water added, and then a clean camel's-hair brush is dipped into the 
mixture and brushed over the surface to be inoculated. With this method it is 
advisable to wet the inoculated part with an atomizer after inoculation, or results 
will be meager. 

(7) Occasionally healthy leaves have been rubbed or dipped in the spore mixture 
described in paragraph 6. 

Gravatt made comparative tests of some of the foregoing methods 
of applying the inoculum. This comparison showed one-fourth 
more infection with method 5 than with method 6, with three 
different species of Ribes. General experience has shown the order 
of efficiency of the methods to be as follows: 1, 5, 2, 6, 3, 7, 4. This 
is probably largely due to the greater number of spores used by the 
more successful methods. All will give good results when the 
relative number of spores used is considered. 

With all these different methods of applying spores and moisture, 
it is essential to supply all the water possible without having it 
form large drops and run off. 

As a supplement to the local moistening, it is necessary to keep 
the inoculated plant in a moist chamber for 12 to 24 hours. In the 
experiments by the writer and his associates the preferred method 
has been to keep the plant in the moist chamber 48 hours. 

The matter of a proper moist chamber is a problem of considerable 
moment. Glass bell jars are good, but costly and easily broken. 
A tightly closed wooden and glass chamber of considerable size was 



42 



BULLETIN 957, U. S. DEPARTMENT OF AGRICULTURE. 



tried, but is unsatisfactory because the tender leaves of the inoculated 
plants are liable to scald in hot weather. Hunt (57) tested a form 
of the iceless refrigerator for this purpose. This is a modification 
of the field moist chamber described by Keitt (61, p. 540-541), 
without a continuous water spray. It is essentially a framework 
large enough to receive several potted plants, on top of which a large 
pan of water is placed. Around the framework is fitted a loose 
curtain of heavy cheesecloth completely surrounding the framework 
on the sides and extending from the water in the pan on top to the 
ground. In use, the cloth is wet thoroughly and the water in the 
pan keeps it wet. This keeps the air within the chamber saturated 
with moisture and cool, which is the desired condition for the plant. 
This has been very successful even in the hottest summer weather 
and has the desirable qualities of durability, cheapness, portability, 
and simplicity. 

Clinton (12) has recently reported the successful inoculation of 
plucked leaves of Bibes in moist chambers. This is an old method 
with the rusts, and was used by Barclay in India as early as 1887 (4) . 
Clinton has apparently developed this method to a point of maximum 
efficiency. It has not been used in the investigations by the writer 
and his associates, the preferred method being to retain natural 
conditions as far as possible in making susceptibility tests. 

FACTORS CONTROLLING THE INFECTION OF RIBES. 

Among the factors controlling infection of Bibes by Cronartium 
ribicola may be mentioned moisture, sunlight, age of leaves inocu- 
lated, and age of inoculum. 

Frequent allusions are made by investigators to the need for 
abundant moisture in producing the infection of Bibes by secio- 
spores and urediniospores of Cronartium ribicola and in spreading 
the fungus on Bibes. 

In 1904, Aderhold (1) performed a series of experiments to deter- 
mine the influence of moisture upon the infection of Ribes vulgare 
by asciospores of Cronartium ribicola. He had two inclosed cells, 
the air in one of which was moistened by artificial rain, while in the 
other it was kept relatively dry; he had similarly arranged plats 
open to the free air. The conditions in these cells and plats he 
summarized, as in Table III. 

Table III.- — Conditions in cells and plats of Aderhold' s experiments. 



Experiment. 


Air. 


Temperature. 


Moisture. 


Amount 
of light. 






High 


Very great 


Small. 


2. Dry cell 


.do 




Slight 


Do. 


3. Open rain plat 






Great 


Great. 


do 




Normal 


Do. 













WHITE-PINE BLISTER RUST. 43 

Aderhold placed his experimental plants in these cells and plats 
on April 16. On May 6 all of the plants were heavily dusted with 
geciospores, and half of those in each cell and plat were put under 
conditions opposite to those they were in before inoculation. The 
plants from the closed rain cell when inoculated and replaced in the 
same cell took the disease heavily. Those from the closed dry cell 
when inoculated and placed in the closed rain cell also took the 
disease heavily. Those transferred from both the rain and dry 
cells to the dry after inoculation showed no infection. All the plants 
kept in the open plats failed to take the disease. It is apparent 
from his results that infection depends upon an atmosphere that is 
nearly saturated with moisture. 

Experience in the greenhouse has shown that it is necessary to 
have abundant moisture on the leaf surface for infection to succeed. 
The leaf itself must be wet, without having large drops of water 
collect. This moisture must be retained for some time by keeping 
the surrounding air saturated with water vapor. Gravatt made a 
series of parallel tests, part of the inoculated plants being kept under 
bell jars 2 hours, part of them 7 hours, and another part 24 hours. 
Infection occurred with the 7-hour and the 24-hour plants, but not 
with the 2-hour tests. The writer made a series of inoculations in 
the greenhouse with seciospores without putting the plants in moist 
chambers. Not one infection resulted within 14 days, the usual 
time necessary to reach full fruiting condition. The plants were 
then put into moist chambers for 48 hours, and fair infection resulted 
from the spores put on the leaves 14 days before. Ewert (37) per- 
formed a similar experiment in 1912 with the same results, as did 
Werth (177) in 1915 and York 25 in 1919. McCubbin in 1916 inocu- 
lated two leaves on each of seven shoots of a Ribes nigrum plant. 
Two of the shoots were put in a moist chamber for 48 hours. The 
remaining five shoots were left uncovered. The leaves of only the 
two inclosed shoots developed infection. 

It is equally necessary to have plentiful moisture for infection to 
occur out of doors. Hennings (53) found a severe outbreak of the 
disease on Ribes during a dry time in the Dahlem Botanical Garden, 
but he attributes the intensity of the attack to the daily watering 
(sprinkling) of the bushes. Ewert (35) says, "In the summer of 
1902, moisture, the important factor for infection, was not lacking, 
so all conditions were favorable for the spread of the Cronartium. "' 
Schellenberg (123) attempted to inoculate Ribes bushes with secio- 
spores from Finns cembra in the open air. He attributes his failure 
to produce infections to the bright sunny weather prevailing at the 
time. In 1910, the writer (131) inoculated Ribes leaves with fresh 

25 York, H. H. Op. cit. 



44 BULLETIN 957, U. S. DEPARTMENT OF AGRICULTURE. 

seciospores out of doors. The leaves were not wet, but there was 
dew every night. No infection resulted. 

In 1913, Stewart and Rankin (151) made some observations on 
this problem. On May 14 two white-pine trees were found bearing 
abundant open secia. On May 17, they were cut down and burned. 
About 120 feet from the pines there was a small plantation of Riles 
nigrum and R. vulgar e. The weather was dry and unfavorable for 
infection of the currants to take place for several days before May 
15. The forenoon of May 15 was damp, but in the afternoon it 
dried off and remained dry until after the trees were destroyed. 
They concluded that the infection of Ribes which developed on June 
10 apparently could have taken place only in the wet forenoon of 
May 15. It appears to the writer that the long incubation period 
indicates that the spores from the pines stuck to the Ribes leaves 
without germinating until a later rainy period long enough for 
infection to occur. 

Studies made by Pennington 26 and Snell (145) in New York in 
1918 on Ribes rotundifolium show the absolute dependence of the 
spread of this fungus upon moist weather. They found that six 
distinct generations of urediniospores were produced during the 
season, with a slight seventh one the last of the summer. These 
appeared approximately two weeks after spells of rainy weather. 
York working in the White Mountain region for three years has 
made a great many successful field inoculations on various species 
of Ribes without using any form of moist chamber. His work was 
largely carried on, however, in localities naturally moist. 

Temperature also is an important factor. Probably much of the 
efficiency of Hunt's iceless refrigerator inoculating chamber is due 
to the rather low temperature obtained. Cronartium ribicola is 
favored by low temperatures, as is shown by the optimum tempera- 
tures determined for it by Doran (28) . Doran also inoculated plants 
of Ribes which were then kept at 3°, 12°, and about 22° C. No 
infection occurred on the first and last, while the one at 12° C. devel- 
oped uredinia. 

Sunshine is an important factor, indirectly if not directly. Its 
direct influence upon the spores is destructive (30, 88) but it is doubt- 
ful if this action is powerful enough to hinder germination greatly 
if sufficient moisture is present. Indirectly sunshine affects in- 
fection by quickly reducing moisture. It seems that a moderately 
cool temperature is most favorable and that bright sunlight may 
elevate the temperature above the optimum for the fungus. 

The size of the leaves alone seems to have little or no influence 
upon their susceptibility to infection (147). Leaves as small as 

26 Pennington, L. H. Op. cit. 



WHITE-PINE BLISTER RUST. 45 

4 mm. wide have been found bearing groups of uredinia (PL IV, 
%. 2). 

The age and relative maturity of the leaf has much to do with its 
susceptibility. It has been the general experience that Kibes leaves 
may be overmature and also may be too young to take the disease. 
Infection does not occur on the leaves of a given species of Ribes 
until they have reached a certain degree of maturity. Leaves 
produced by buds developing in late summer or fall, even if very 
small, readily become infected. The different species of Ribes vary 
much in this regard. Ribes nigrum shows a great range in its age 
of susceptibility, while resistant species become infected only on 
leaves of a certain maturity. The most favorable stage of growth 
seems to be about when the leaf attains full size but has not become 
hardened and leathery as it does later. Tests were made by Gravatt 
in 1915 in the greenhouse on Ribus nigrum. The plant had three 
shoots of nearly equal size and development. They bore fully 
mature leaves at the base and had leaves at the tips about half 
grown. In this case all the leaves became infected except the lower 
three or four on each shoot. In 1916, McCubbin (ms. report) made 
several series of inoculations with asciospores upon Ribes nigrum 
leaves of various ages. The plants were not kept under the best 
growing conditions, so the results are less pronounced than might 
otherwise be expected. He produced no infection on the youngest 
leaves. The older ones took the disease, but the overmature ones 
took it least of all. York 27 made greenhouse tests with plants of 
Ribes nigrum, R. triste, R. glandulosum, R. Mrtellum, and R. lacustre. 
Leaves of various ages were present on all the plants. The mature 
ones showed infection first. The degree of infection was heaviest 
on the first species and decreased in the order named, R. lacustre 
having but two pustules on a single leaf. Later, the half-mature 
leaves of R. nigrum and the leaves of R. triste and R. glandulosum 
two-thirds mature became infected. The younger leaves did not 
become infected then, but when reinoculated later they took the 
disease, except that those of R. lacustre remained healthy. In most 
inoculation tests made by the writer and his associates in the green- 
house both the oldest and the youngest leaves remained free from 
disease, although they were treated exactly like the others. York 27 
tested this point extensively in the open in 1918 and found that 
leaves just unfolding were almost invariably immune to the fungus; 
older ones took the disease readily; and overmature ones (especially 
late in the season) were immune. Pennington 28 reached similar 
conclusions working independently. 

27 York, H. H. Op. cit. 

28 Pennington, L. H. Op. cit. 



46 BULLETIN 957, U. S. DEPARTMENT OF AGRICULTURE. 

Snell 29 made inoculations out of doors with spores from unopened 
secia on April 30, 1918. He inoculated opening buds of Ribes glandu- 
losum by carefully inserting a knife so as not to injure the leaflets and 
inserting the spores between the folds of the leaves. The largest 
leaves were 3 cm. broad, the smallest 3 to 5 mm. long. It was very 
rainy, so there was plenty of moisture. No infection was visible on 
May 15, but on May 22 heavy infection was present on all the leaves 
inoculated. The leaves on this date ranged from nearly full size to 
those just opening. The infection was heaviest on the largest leaves 
inoculated and decreased to a light infection on the smallest. The 
check plants were healthy. There are two possible factors which 
might have delayed the infection a week longer than usual. These 
are cool temperature and the immaturity of the leaves. The experi- 
ence of the writer leads to the belief that the latter was the principal 
factor involved in this case. Later Snell found natural infection on 
leaves of Ribes vulgare that were only 12 mm. wide. 

It has been noted repeatedly that the earliest infections on Ribes 
leaves in the spring are about a month later than the time when the 
first seciospores are set free. It is a question whether this is due to 
very cool nights or to the immaturity of the Ribes leaves at this time. 
Noninfection of immature leaves in the greenhouse leads the writer 
to suspect that the latter is the main factor involved. European 
writers (63, 101) have stated that Cronartium ribicola is decidedly 
earlier than the native pine-stem Peridermiums. 

Observations made by Gravatt at Block Island show that new in- 
fection in midsummer was present upon the fifth to the eighth leaves 
from the tip, not counting those less than 5 mm. wide; that is, on 
leaves just mature but not hardened. 

Gravatt and York and Overholts made many inoculations of 
leaves, petioles, and stems with wounds, but found no evidence that 
infection was favored by wounds. 

Whether viability of spores of Cronartium ribicola in culture solu- 
tions, water, etc., is a reliable index of their infective power (43) is a 
question which has arisen more or less insistently since inoculation 
experiments began. Klebahn (70) made a definite test with refer- 
ence to this question with seciospores of Cronartium ribicola. The 
spores were collected on March 20 and kept dry until May 8 when the 
test was made. Some were sown on the leaves of Ribes aureum, some 
were sown on a cover glass coated with a thin layer of sterile Ribes- 
decoction agar, and others were sown on a cover glass moistened with 
water. The cover glasses were kept in a moist chamber to prevent 
drying. The Ribes plants became infected after 12 days on every 
leaf inoculated. The spores on Ribes-decoction agar germinated 

29 Snell, W. H. Period of exposure and size of Ribes leaves infected by the blister-rust fungus. Seen in 
manuscript. To be published in Phytopathology. 



WHITE-PINE BLISTER RUST. 47 

slowly at first, but later developed strong germ tubes in considerable 
quantity. Scarcely a spore in water germinated. As a result of his 
extensive experience with the rusts, Klebahn says: 

... in other words, I believe it possible for spores which do not germinate in 
water to infect leaves of the host plant, and it seems to me to be expedient to distin- 
guish between "infection power" and "viability" of spores more sharply than is 
ordinarily done. 

Gravatt had experiences somewhat similar to the above in his in- 
oculations with urediniospores of Cronartium ribicola in 1917. So 
pronounced has been our general experience in this regard, that many 
germination and longevity culture tests made in 1918 and 1919 were 
duplicated by check inoculations on favorable hosts so far as possible. 

LOCATION OF THE INFECTIONS ON RIBES PLANTS. 

SORI ON THE LEAVES. 

The usual place for uredinia. and telia of Cronartium ribicola to 
form is on the lower side of the Bibes leaf blade. It is rather excep- 
tional for them to appear elsewhere. Nevertheless, they are occa- 
sionally found on the upper side of the leaf blade. They have been 
noted there by Gravatt in the greenhouse and by several out-of- 
door workers on Block Island. The following species have been seen 
with uredinia or telia on the upper leaf surface: Ribes alpestre, R. 
aureum, R. cereum, R. fasciculaturn, R. fontenayense, R. hirtettum y 
R. odoratum, and also the horticultural varieties R. aureum var. 
Utah Yellow and R. vulgare var. White Imperial. It must not be 
concluded that because fruiting bodies are found on the upper surface 
of leaves that the infection took place there. On the contrary, in 
every case seen, it was very evident that the fungus had attacked 
the infected leaf beneath, and the attack had been so intensive that 
some sori were pushed through to the upper surface. There never 
were as many sori on the upper surface as there were on the lower 
one, nor were they so old. 

Some inoculations have been made in Europe to determine if infec- 
tion may take place on the upper surface of Ribes leaves. So far 
as known to the writer they are here summarized: 

In 1913 Ewert (37) brought four potted plants of Ribes nigrum 
into the greenhouse. On April 10 he inoculated the leaves of one 
branch of plant 1 on the lower surface only with fresh seciospores. 
The plant was inclosed in a glass cylinder as a moist chamber. 
Another branch was used as a check. On April 28 the inoculated 
branch bore uredinia upon 11 leaves. On April 15 he inoculated 
the leaves of a third branch, but did not inclose it in a moist chamber. 
On April 26 there was no sign of infection, and it was then inclosed 
in a moist chamber. On May 20 all the inoculated leaves bore 
uredinia. The control remained healthy. 



48 BULLETIN 9*57, U. S. DEPARTMENT OF AGRICULTURE. 

Plant 2 was treated exactly like plant 1 except that the inoculations 
were made on the upper sides of the leaves. No infection resulted. 

Plant 3, on April 10 and 15, was inoculated on one branch on only 
the lower surfaces of the leaves. Another branch was inoculated on 
the upper surfaces only. A third branch was left as a check. Weak 
infection resulted on the first branch. A repetition on April 29 with 
fresh seciospores gave better results. 

Plant 4, left untreated, was in another inclosure of the greenhouse. 
It remained healthy, showing that infection had not occurred before 
the plants were brought into the house. 

On May 26 similar inoculations were made by Ewert with uredinio- 
spores on the upper sides and lower sides of leaves. Infection 
resulted in the latter case and also a slight infection of the lower 
surface of one leaf which was inoculated on the upper surface. 

Another similar series of inoculations made by Ewert on June 6 
gave infection only on the leaves inoculated on the lower side. He 
fails to say in all cases that the sori formed only on the lower surface 
of the leaves, but his language implies that this is the case. Attack- 
ing the problem in another way, Ewert (37) sprayed Rites nigrum, R. 
aureum, and R. rubrum, leaves, part on the lower side only, part on 
the upper side only, and part on both sides, while controls were left 
unsprayed. The details are presented on pages 77 to 79. Because 
some leaves which were sprayed on the upper side developed a few 
sori beneath, he appears to be a little doubtful whether infection 
may occur on the upper surface, but he concludes that it " apparently 
almost exclusively takes place on the lower surface of the leaf." 

The writer and his associates have made hundreds of inoculations 
on the upper surface of leaves of many species and varieties of Ribes, 
without a single direct infection occurring there. Numerous in- 
stances have been noted in these experiments in which infection ap- 
peared on the lower surfaces of leaves that had been inoculated on 
the upper side. This is believed to be due to spores reaching the 
lower surface in some unknown way. In fact it is very difficult if 
not impossible to guard against this. York and Overholts inoculated 
leaves of Ribes glandulosum on the upper side, both with and without 
ring cells, to prevent the spores reaching the lower surface. Slight 
infection occurred on the lower side in some cases where cells were 
not used. Where the cells were used no infection occurred. These 
tests were made on leaves of different ages on plants of various ages, 
from young seedlings up to fruiting bushes. Tubeuf (173) inoculated 
leaves of Ribes nigrum on the upper side by applying the spores in 
water with a brush. All of the leaves thus inoculated remained 
healthy except a single one which had a uredinium on the lower sur- 
face. He was uncertain whether a spore infected it through the lower 
surface or through a lesion on the upper surface. 



WHITE-PINE BLISTER BUST. 49 



SORI ON COTYLEDONS. 



The cotyledons of young Ribes seedlings are apparently quite sus- 
ceptible to infection by asciospores and urediniospores of Cronartium 
ribicola. Relatively heavy infection has resulted from inoculations 
on the lower surface of cotyledons of Ribes americanum, R. missouri- 
ense, R. oxyacanthoidas, R. rotundifolium, R. glandulosum, and R. 
fasciculatum seedlings in the greenhouse and on R. glandulosum in the 
field. 

SORI ON FLORAL BRACTS AND BUD SCALES. 

Infection was secured by Gravatt from inoculations of the floral 
bracts of Ribes aureum in several different cases in the greenhouse. 

Infection of opening buds and of bud scales merits more investiga- 
tion. McCubbin (85) suggested the possibility of infection of partially 
open buds in the fall and the overwintering of the fungus, but he 
could not prove that it occurs. Infection of young leaves scarcely 
out of the bud occurs, but it seems to be limited te leaves that are 
relatively mature, though small. (See discussion on pp. 44 to 46.) 
Search for infections of buds on heavily infected Ribes nigrum bushes 
failed to reveal any infections (151, p. 44) . Gravatt made inoculations 
of buds about to open, but with no success. York 30 has inoculated 
successfully an inner bud scale of Ribes nigrum. 

SORI ON PETIOLES. 

Next in frequency to infection of the lower surface of the leaf 
blade is infection of petioles. The first published account of this, 
so far as the writer knows, was given in 1912 (133, 134, 135) and 1913. 
At that time it was considered to be very uncommon. Since than 
a considerable number of such cases have been noted both in the 
greenhouse and out of doors. The following species have developed 
uredinia or telia, or both, upon petioles, as many as 25 or more 
petioles on a single plant being thus attacked: Ribes americanum, 
R. aureum, R. bracteosum, R. cereum, R. culverwellii, R. cynosbati, 
R. divaricatum, R. erythrocarpum, R. fasciculatum, R. giraldii, R. 
glandulosum, R. inerme, R. lacustre, R. nevddense, R. nigrum, R. 
parishii, R. petraeum, R. robustum, R. setosum. The following 
cultivated varieties have had petiolar attacks: Ribes nigrum hort. 
vars. Black Victoria, Climax, and Seabrook Black; R. reclinatum 
hort. vars. Berkeley, Golden Prolific, Poorman, Transparent, and 
Van Fleet; R. vulgare hort. var. White Imperial. Some 10 or 12 
as yet unidentified species collected by Beattie in the Rocky Mountain 
and Pacific coast regions have exhibited the same phenomenon. 

30 York, H. H. Op. cit. 

46103°— 21— Bull. 957 4 



50 BULLETIN 957, U. S. DEPARTMENT OF AGRICULTURE. 

In many cases the petiole became diseased by growth of the 
mycelium downward from the leaf blade into it, but direct infection 
of the petiole occurs occasionally. This is shown by the presence 
of infections on- the petiole one-half inch or more distant from other 
infections. Microscopic examination by Colley in such instances 
has shown the intervening tissues to be entirely free from migrating 
mycelium. York 31 had several instances where infection took place 
well down. on. the petiole, and no other infection was present either 
on that petiole or the leaf blade. While many inoculations of petioles 
have been made by members of the Office of Investigations in Forest 
Pathology, but few have been successful, as above indicated. 



SORI ON STEMS. 



Evidence of infection of Kibes stems has long been sought In 
1917, Posey, Gravatt, and Colley (112) discovered three uredinia 
on young stems of Riles Urtellum which resulted from natural in- 
fection in an outbreak area. Artificial inoculations on young stems 
of the same species with aeciospores produced 18 more uredinia. A 
single stem infection was produced by Gravatt in the greenhouse 
upon a young seedling of R.fasciculatum (PL V, fig. 2). ^ciospores 
were used in this case also. While the tender stem was completely 
girdled, it survived long enough to form wood and finally completely 
outgrew the disease. Since then Taylor has successfully inoculated 
with aeciospores the stems of young seedlings of Riles missouriense 
and of R. amencanum in the greenhouse. York has infected stems 
of young R. glandulosum plants with aeciospores and urediniospores 
out of doors, and has found natural infections on the same species 
and on R. cynoslati. He has infected a young stem of a 2-year-old 
plant of R. cynoslati with aeciospores in the greenhouse. 

RELATION OF STOMATA TO THE INFECTION OF RIBES. 

A number of investigators of the Uredinales have stated that 
seciospore and urediniospore germ tubes obtain entrance to their 
hosts through the stomata (34, 70, 110, 149, 171). 

As heretofore stated, Oronartium rilicola 'infects the Ribes plant 
on the lower side of the leaf mostly. Less frequently it infects the 
petioles, floral bracts, and cotyledons. It may infect young stems. 
Infection never occurs on the upper surface of the leaf. Examina- 
tion of a number of different species of Ribes has been made by mem- 
bers of the Office of Investigations in Forest Pathology. Data on 
the stomata may be summed up as follows : 

Stomata were present in large numbers on the lower surface of leaves of all species 
examined. Stomata were present in small numbers on the upper surface of leaves of 
Ribescerm m (60,78),iZ. inebrians (60, 78), R. odor atum by Marshall, R. orientale (160), and 

31 York,H.H. Op.cit, ~~ 



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WHITE-PINE BLISTER RUST. 51 

R. vulgar e by Marshall. None were found by Unger (176) on the upper surface of 
leaves of R. alpinum nor by Taylor on R. americanum, R. aureum, R. carrierii, R. 
culvericdlii, R . fasciculatum, R. nigrum, R. reclinatum, R. speciosum, and R. (tenui- 
florum) aureum. Stomata were found only on the lower surface of cotyledons of R. 
fasciculatum and R. missouriense, the only ones examined by Taylor. Stomata were 
found by Taylor to be not uncommon on petioles of R. americanum, R. aureum, R. 
carrierii, R. culverwellii, R. hirtellum, R. inerme, R. nigrum, R. odor alum, R. reclina- 
tum, R. sanguineum, R. speciosum, R. succirubrum, R. (tenuiflorum) aureum, and R. 
vulgare. None were found on petioles of R. curvatum and R. fasciculatum. Janczewski 
(60) states that stomata are present on the young stems of R. petraeum. A few stomata 
were found on young stems of R. aureum, R. hirtellum, R. nigrum, R. odoratum, R. 
reclinatum, R. succirubrum, R. (tenuiflorum) aureum, and R. vulgare. None were found 
on the stems of R. carrierii and R. fasciculatum. These findings compare well with 
the inoculation results, if stomata are the avenue for infection. It is perhaps to be 
expected that infection may be produced on the upper surface of the leaves, but only 
very rarely. Colley (20) found young uredinia forming in the substomatal spaces, 
which would indicate that infection took place in that vicinity and presumably 
through the stomata. York 32 found germ tubes of aeciospores entering the stomata of 
leaves of Ribes cynosboti, R. glandulosum, and R. nigrum. 

VARIATIONS IN APPEARANCE ON RIBES LEAVES. 

The study of great numbers of Ribes leaves infected in the green- 
house and of very numerous specimens of diseased leaves collected 
in the field from Maine to Minnesota, during the past ten years, has 
revealed some distinct variations in the appearance of the fungus 
and of the diseased leaves of different species and varieties of Ribes. 
Previous study of such differences seem to have been made chiefly 
by Hennings (52, 53) . 

BLISTERY APPEARANCE OF THE UREDINIA. 

In August, 1916, there occurred a very hot, dry period in New 
England. This was followed by the finding of a few very peculiar- 
looking uredinia on Ribes nigrum and R. vulgare. Under a hand lens, 
the uredinia were not of the usual mealy appearance, but looked more 
like tiny blisters. Examination showed that the epidermis of the 
leaves had become toughened, so that the uredinia did not burst 
through it, as they usually do, but pulled it loose from the inner leaf 
tissues and in this way actually formed small blisters. The uredinio- 
spores never broke through it. From the weather conditions pre- 
ceding and at the time this occurred it is believed that the dry, hot 
weather rendered the epidermis tougher than usual. It is a rare 
occurrence, as only a few cases have been noted. In 1916, one speci- 
men came from each of the States of Massachusetts, Maine, and Ver- 
mont. In 1917, a single case was found in New Hampshire. Hedg- 
cock noted this in 1919 upon artificially inoculated Ribes bushes on 
Block Island. Recently, blisters have been noted on R. auruem and 
R. fasciculatum in the greenhouse. 

aa Fork, H. H. Op. cit. 



52 BULLETIN 957, TJ. S. DEPARTMENT OF AGRICULTURE. 

DISTRIBUTION AND SIZE OF THE SORI AND COLONIES. 

It has been found that Cronartium rihicola forms its uredinia and 
telia upon each species of Bibes in a manner which in general is 
characteristic of that host species (Pis. IV; V, figs. 1, 3. and 4 : and VI). 
This was noted in 1902 by Hennings (52, 53), who described some of 
the more striking variations. Like all general statements, the fol- 
lowing are subject to individual variations from the normal or average 
condition for the host species. A statement by Hennings (52, p. 130) 
indicates the degree of variation discernible to a keen observer: 

It is especially noticeable that the fungus, according to the character of the leaves 
of various species of Ribes, shows great variations in the form and color of the spots 
produced on the leaves, the form and size of the sori, and the size of the telial colu- 
mella, so that a new observer would assume that several of the fungus forms were 
specifically distinct. 

In general, on species which are closely related, or which closely 
resemble each other, the fungus behaves in a similar manner. Some 
of the more striking variations are described as follows: 

Ribesia. 33 — Ribes petraeum and var. atropurpureum: Sori close together and evenly 
distributed over the spots; spots large, soon overrunning a large part of the leaf 
surface. 

Ribes rubrum vars. petrowalskyanum, pubescens, and sibirica: Very scant 
sori, located beside large veins of leaf; var. scandicum has abundant sori, close 
together, generally distributed. 

Ribes triste: Sori clustered on definite spots; spots small, widely separated. 

Ribes vulgare: Sori not plentiful, clustered on spots; spots small and isolated 
(PI. IV, fig. 3; V. fig. 4). 
Heritiera. — Ribes coloradense: Sori thickly clustered on diffuse spots. 

Ribes glandulosum: Sori usually thinly scattered over large diffuse spots or 
entire leaf surface, telia very slender and long (PI. VI, fig. 1). 

Ribes erythrocarpum: Sori plentiful, clustered. 
Calobotrya. — Ribes glutinosum: Sori thickly clustered in local areas. 

Ribes nevadense: Sori thickly grouped on local spots; spots large and distinct. 

Ribes viscosissimum: Sori isolated, scattered over entire leaf surface. 
Symphocalyx. — Ribes aureum and vars. tenuifiorum and palmatum: Sori abundant, 
closely grouped in rather distinct spots which usually are well separated from 
each other (PI. IV, fig. 1). 

Ribes odoratum: Sori abundant, closely grouped in spots usually well sepa- 
rated from each other. 
Arophyllum. — Ribes cereum: Sori clustered on definite rounded spots which soon 
die. 

Ribes inebrians: Sori on rounded spots. 
Eucoreosma. — Ribes americanum: Sori sparse and scattered, on small irregular spots 
which redden and die; heavy infection rare; telia short, one-half to 1 mm. 

Ribes bracteosum: Sori plentiful, on large diffused spots or patches of leaf 
surface. 

Ribes nigrum (PI. IV, fig. 4) and vars. aconitifolium, fasciculatum, "folio 
argentea": Sori e crowded densely, often over entire leaf surface, vigorous; telia 
abundant, reaching 2 mm. in length. 

Ribes viburnifolium: Sori scant, dead spots form early. 

33 The species of Ribes are grouped according to the arrangement of Janczewski (60). 



WHITE-PINE BLISTER RUST. 53 

Grossularioldes. — Ribes lacustre: Dead spots formed early on infected leaves; 

sori sparse or diffuse, irregular spots; telia rather scattered. 
Grossularia. — Ribes lobbii: Sori thickly crowded on rounded spots, telia well de- 
veloped. 

Ribes menziesii: Sori sparse on small irregular spots. 

Ribes speciosum: Sori seated closely together on large rounded spots, telia 

rather short. 
Eugrossularia. — Ribes alpestre: Sori thickly crowded on large rounded spots, 
telia short. 

Ribes curvatum: Sori crowded on rounded spots on young leaves, on irregular 
spots on old leaves; telia quite plentiful; spots with reddened edges late in 
season. 

Ribes cynosbati: Sori plentiful on definite spots, which are usually rounded 
(PI. V, fig. 1); spots sometimes wedge shaped, lying between two branching 
veins of the leaf; telia crowded in small groups, 1 to 1| mm. long, rarely over 
entire leaf surface. 

Ribes divaricatum: Sori scattered on rounded spots. 

Ribes missouriense: Sori crowded on rounded indefinite spots on young leaves, 
densely crowded on small leaves, dead irregular spots on old leaves. 

Ribes leptanthum: Sori very scant; dead spots appear so early that uredinia 
can form with difficulty, often only one uredinium on a spot, many spots with- 
out sori; produces the least sori of any species yet noted. 

Ribes hirtellum: Sori usually crowded thickly on small spots; uredinia on 
small rounded spots; telia small, crowded densely; on older leaves, on small 
indefinite, irregular spots bounded by veinlets; spots sometimes purplish on 
the edges. 

Ribes oxyacanthoides: Like R. hirtellum. 

Ribes reclinatum: Sori rather sparse, on small irregular spots of dead tissue. 
There is a tendency toward a reddening or purpling of the edges of the spots 
on old leaves. (PI. V, fig. 3.) 

Ribes rotundifolium: Sori crowded on small irregular spots. The spots die 
early and are likely to become reddish around the edges even on rather young 
leaves. It is rather rare for the entire leaf to become covered with sori. 

Ribes selosum: Much like R. cynosbati. 
Hemibotrya. — Ribes fasciculatum vars. chinense and japonicum: Sori densely 

crowded on large rounded spots. 
Diacantha. — Ribes diacantha: Sori on rounded spots. 

Ribes giraldii: Sori scattered on rounded spots. 

In general, it may be said that R. nigrum and its varieties is the 
optimum host species among the Ribes for Cronartium ribicola. 
Ribes aureum and R. odoratum and their varieties are perhaps next 
to R. nigrum in favoring the growth of the fungus. R. reclinatum 
does not take the disease readily, but is by no means immune to it. 
In fact, no species or variety yet fully tested is entirely immune. 
R. leptanthum probably produces fewer sori for the extent of infection 
than any other species. This is caused by the very early death of 
the infected tissue. 

PALE COLOR OP INFECTED SPOTS ON THE LOWER SURFACE OF RIBES LEAVES. 

The production of sori on an infected spot on the lower surface of 
Ribes leaves is often preceded for one, two, or three days by a pale 



54 BULLETIN ■ 057, U. S. DEPARTMENT OF AGRICULTURE. 

coloration (PL VI, fig. 2). This is most likely to occur on young 
leaves. It is a fairly certain symptom of a successful inoculation. 

Striking cases of the preservation of the normal green color in the 
infected areas, while the rest of the leaf is etiolated, have been noted 
a number of times. 

PALE COLOR OF INFECTED SPOTS ON THE UPPER SURFACE OF RIBES LEAVES. 

Numerous young Bibes leaves inoculated in the greenhouse devel- 
oped pale spots on the upper side of the infected leaves (PL V, fig. 1). 
These varied from nearly white to yellow. Similar spots are often seen 
in the field. Some species of Ribes seem to be more likely to develop 
these spots than others. Among those showing this etiolation as a 
result of infection by Cronartium ribicola may be mentioned: Ribes 
aureum and its varieties, R. cereum, R. cynosbati, R. divarication, R, 
eryihrocarpum, R. fasciculatum, R. glandulosum, R. Mrtellum, R. 
inebrious, R. inerme, R. leptanthum, R. nevadense, R. nigrum, R. 
odoratum, R. reclinatum, R. rotundifolium, R. setosum, R. speciosum, 
R. triste, and R. vulgare. These spots seem to be produced by condi- 
tions existing in the immature Ribes leaf when attacked by the 
fungus very actively. 

REDDENING OF INFECTED SPOTS ON RIBES LEAVES. 

Certain species of Ribes react to infection by Cronartium ribicola 
by a reddish or purplish coloration around the edges of the infection. 
This is common with some species, while others apparently must 
have the leaves at a certain stage of maturity for this coloration to 
occur. Ribes americanum, R. curvatum, R. rotundifolium, R. viburni- 
folium, R. glandulosum (old thick leaves), R. cynosbati (old thick 
leaves) , R. Mrtellum (old thick leaves) , R. oxyacanthoides , R. reclina- 
tum, and R. missouriense develop the red color in the order named. 

DEAD AREAS OF INFECTED LEAF TISSUE. 

Where the attack of Cronartium ribicola is intense, areas of the 
oldest infected tissue of the diseased leaves collapse and die (PL V, 
figs. 3 and 4 ; PL VI, fig. 3) . The different species of Ribes vary much 
in this respect, some developing dead spots very quickly and some 
doing so only after considerable time. Ribes nigrum usually resists 
death tenaciously. When a spot dies it is commonly a large one and 
soon results in the premature fall of the affected leaf. At the other 
extreme is R. leptanthum. With this species the infected spots die 
very quickly, even before uredinia can form (147). Less than 10 per 
cent of these spots produce any uredinia or telia, and those few spots 
bearing sori usually have only from one to several stunted sori. All 
species of Ribes tested have sooner or later developed dead spots. 
Just how much secondary fungi contribute to the killing . of host 
tissues is entirely unknown. 



Bui. 957, U. S. Dept. of Agriculture. 



Plate VI. 



sri : t'i V* 1 












Leaves of Ribes Infected with Cronartium ribicola, Showing Different 

Types of Infection. 

Fig. 1.— Infected leaves of Ribes glandulosum, showing lower surfaces; the sparsely scattered telia 
are well distributed over the entire surface. Etiolation caused by the disease is also evident. 
X §• Fig. 2. — Lower surfaces of leaves of Ribes sp., showing etiolated spots where infection has 
taken place, two days before uredinia were formed. X f . Fig. 3. — Lower surfaces of infected 
leaves of Ribes aureum, showing the disease distributed in local spots mostly well separated from 
one another. On the right side of the right Jeaf a large area of leaf tissue has died. Etiolation 
from the disease is evident. X f . 



WHITE-PINE BLISTER RUST. 55 

THE UREDINIA AND UREDINIOSPORES. 

GENERATIONS OP UREDINIA. 

In 1918, Pennington 34 and Snell investigated the number of genera- 
tions of uredinia of Cronartium ribicola produced in the Adirondack 
region, and the weather conditions that might influence their pro- 
duction. Ribes rotundifolium, R. cynosbati, and R. glandulosum 
were the principal species used in these investigations. The observa- 
tions were made in four different localities within 10 miles of the 
town of Lewis. There were seven periods of uredinial production in 
1918. The first generation began on May 28, reached its climax 
about June 12, and then gradually fell off until June 26 to 28, when 
the second appeared. The third began to appear about July 12 and 
reached its maximum on July 19 to 22. The second and third crops 
of uredinia were located almost entirely on those leaves which were 
originally infected by seciospores or those adjacent to them. Drought 
from July 18 to July 28 caused most of the infected leaves to drop 
from the bushes of Ribes cynosbati and R. rotundifolium, leaving them 
partly or entirely defoliated. The fourth crop was much smaller, 
but well marked, and came on August 19 and 20. The fifth genera- 
tion came on September 12 to 15; it would have been more abundant 
had not a heavy frost on September 11 killed all the leaves of R. 
glandulosum and some on the other two species of Ribes. The sixth 
crop appeared especially on the second crop of leaves of the earlier 
defoliated bushes and on fresh green leaves of bushes in sheltered 
situations. The seventh generation appeared on October 15 to 18 
on leaves ready to fall. A study of the weather conditions, as noted 
at Lewis, showed that about two weeks before the appearance of each 
new generation there was a period of at least 24 hours of rainy and 
cloudy weather. But not all such periods were followed by new 
crops of urediniospores. 

In 1919, Pennington 34 found that the generations of uredinia were 
not as distinct at Lewis, N. Y., as they were in 1918. The first four, 
on May 23, June 21 and 22, July 3, and July 20, respectively, were 
well defined. A fifth on August 7 and a sixth on August 21 were 
distinct on some bushes, but in most places overlapped. In general, 
after August 1, the generations overlapped, because of rain every 
day or two, so as to become confused with each other. 

SEASON OF PRODUCTION OP THE UREDINIA. 

Like the ascial season, the beginning of the uredinial season of pro- 
duction varies with conditions somewhat, though to a less marked 
degree. May 16 is the earliest recorded date for mature uredinia. 
A week after this is the more usual time for them to be found in most 

s< Pennington, L. H. Op. cit. 



56 BULLETIN 957, U. S. DEPARTMENT OF AGRICULTURE. 

localities. (For range of observed dates in various sections of North 
America, see Table V, p. 72.) 

In the White Mountain region of New Hampshire, York 35 found 
fresh urediniospores in 1918 on May 16 and as late as November 17. 
Urediniospore production there continued on some bushes for 185 
days, while under average conditions it continued about 120 days. 
At this place the following species were under observation: Ribes 
cynosbati, R. glandulosum, R. lacustre, R. nigrum, R. odoratum, R. 
oxyacanthoides, R. reclinatum, R. triste, and R. vulgare. Uredini- 
ospore production continued the longest time (185 days) on R. nigrum 
and the shortest time (65 days) on R. lacustre. In general, it can 
be stated that the urediniospores continue to form as long as there 
are susceptible leaves on the Ribes bushes of a given locality. 

York 35 found that the maximum urediniospore production in 1918 
occurred about July 14 to 16 and in 1919 about July 25 to 26. After 
these dates came the maximum sporidia production, and then the 
bushes became almost completely defoliated. 

DISTANCE OF DISSEMINATION OP THE UREDINIOSPORES. 

In the early work upon Cronartium ribicola in North America the 
wide dissemination of the fungus from a given center appeared to 
take place by means of the urediniospores. Stewart and Rankin 
(151), who had an especially good opportunity to study this point, 
concluded that the urediniospores were blown at least one-half mile. 
Early general observations of the spread of this stage indicated that 
a wet season greatly favored it, while a dry season just as markedly 
retarded it. 

McCubbin (87) found that urediniospores fall in still air about 8 
feet in 5 minutes. He calculated that a 30-mile breeze would carry 
them 2\ miles in this time. Theoretically they may be distributed 
long distances if located on a hill or if picked up by convection air 
currents. But most of these spores are actually produced within 2 
feet of the ground in most localities, so that they are not picked up 
by the wind as readily as the seciospores, which are commonly pro- 
duced a number of feet above the ground. When set free, the 
urediniospores adhere in masses, so that they are not as readily 
blown by the wind as are the seciospores, which tend to fall apart 
when dislodged from the secium. 

York and Overholts (cited in Spaulding, 145) in 1918 at North 
Conway, N. H., found urediniospores in spore traps up to 300 yards 
distant from the known source. This was where rain was plentiful 
practically all summer. Observations on infections of Ribes glandu- 
losum and R. cynosbati plants indicated that the disease spread by 
urediniospores up to 100 yards. In some cases where the bushes 

& York, H. H. Op. cit. 



WHITE-PINE BLISTER RUST. 57 

were protected by surrounding trees or other objects, the rust spread 
little or not at all. In other words, where moisture was plentiful 
through the season, the distance of spread by urediniospores was 
governed by factors controlling the free access of the wind. ' In 
Essex County, N. Y., drought prevailed through July and August 
in 1918. Here Pennington 36 worked with spore traps and Snell (128) 
gave special attention to a study of the spread of the uredinial stage 
on Ribes. The Ribes of this section were largely Ribes rotundifolium 
and R. glandulosum. Here the rust spread in most instances only 
to adjacent leaves from those first infected on a given bush. The 
spore traps here caught urediniospores only at a very short distance, 
50 feet or less. 

In 1919, York (cited in Spaulding, 146) caught urediniospores up 
to a distance of 3,400 feet in an open location, but they did not 
germinate. Urediniospores caught at 3,200 feet did germinate. 
Pennington 36 caught urediniospores up to 156 feet. 

AGENTS DISSEMINATING THE UREDINIOSPORES. 

Wind has been supposed to be the principal agent distributing 
the urediniospores of Cronartium ribicola. While this supposition is 
correct in the main, other agents are concerned in the matter. 

Hennings (53) says that sprinkling diseased Ribes plants with a 
strong stream of water carries urediniospores from plant to plant. 
Rain accompanied by high wind is known to carry spores of some 
plant diseases (38, 44). It is entirely possible for this to occur with 
any spore capable of wind distribution, as in the present case. 

In 1917 Gravatt and Marshall (45) made studies in the experi- 
mental greenhouse at Washington, D. C. They found that weevils, 
snails, slugs, and sow bugs fed on the uredinia and telia of Cronartium 
ribicola on the diseased plants. The ingested urediniospores lost 
their viability to a large extent, but not entirely. 

In the same year Gravatt and Posey (46), working at Kittery 
Point, Me., found that gipsy-moth larvae feed quite freely upon leaves 
of Ribes Mrtellum and R. vulgare and that in some cases the only 
infected leaves were those which had been partially eaten by insects, 
indicating that they carried the spores which infected the leaves. 
The insects were found carrying viable urediniospores on their bodies. 
There can be no doubt that these insects play an important role in 
the local distribution of this fungus within the gipsy-moth infested 
area. 

Studies by Snell (127) at Lewis, N. Y., in 1918, show that a large 
number of insects visit Ribes plants during the season when the rust 
is present upon the leaves. The spore-laden insects were inclosed 
in chambers with the tips of Ribes glandulosum plants, and infection 

36 Pennington, L. H. Op. cit. 



58 BULLETIN 957, U. S. DEPARTMENT OF AGRICULTURE. 

resulted in due time. Many of these insects are, of course, accidental 
visitors, but quite a number feed or breed upon the Bibes plants. 
All of those which frequent the Bibes bushes by preference may carry 
mariy spores of both uredinia and telia. Such insects would be 
most likely to spread the disease, since upon leaving one Bibes plant 
they would seek another, thus scattering the spores exactly where 
they could start new colonies of the disease. But the accidental 
visitors, in a locality where Bibes bushes are abundant, could also 
spread the spores locally, but in a much more indiscriminate manner, 
so that but a very small percentage of the spores would ever reach 
leaves of Bibes. 

Aside from carrying spores on their bodies, some insects feed 
directly on the uredinia and telia and a few of the excreted spores 
are known to retain their viability. Still other insects may be leaf 
eaters and ingest the rust spores only accidentally. These would be 
of minor importance in spreading the disease. 

At Lewis, N. Y., where the Bibes bushes overhang narrow cattle 
paths which wind through a heavy cover of blackberry, raspberry, 
and other low shrubs, observations by Pennington and Snell indicate 
that cattle, sheep, horses, dogs, and berry pickers may carry the ure- 
diniospores from an infected bush to neighboring healthy ones. 

The remarks on the carrying of aeciospores by automobiles, steam 
trains, and electric cars on page 36 apply also to some extent to the 
urediniospores. 

GERMINATION OF THE UREDINIOSPORES. 

The urediniospores of Cronartium rihicola have been generally 
found to be erratic in germinating. At one time excellent germina- 
tion occurs; at another, none at all. In the greenhouse experiments 
it seems that urediniospores produced in newly formed uredinia have 
greater infective power than those produced later in the same ure- 
dinia. Such early urediniospores seem to give as good results as 
fresh aeciospores. The former are usualty produced in limited quan- 
tities while the latter are usually abundant. This results in a more 
liberal use of the latter, so that a fair comparison of the two is not 
possible from the usual inoculation work. Gravatt made compara- 
tive tests and concluded that aeciospores and urediniospores from 
newly open sori were about equal in infective power. More such 
tests should be made before any conclusive statement is made on 
this point. 

Gravatt and York found that newly matured urediniospores pro- 
duced out of doors were decidedly more viable than older ones. 
This agrees with general experience in making inoculations in the 
greenhouse. 

Gravatt found that cooling the urediniospores on ice stimulated 
germination. Uncooled spores gave 15 to 23 per cent germination 



WHITE-PINE BLISTER RUST. 59 

while ice-cooled ones of the same lot gave 25 to 54 per cent germina- 
tion. Marshall found the same stimulating effect from a tempera- 
ture of 23° F. upon urediniospores on Ribes leaves stored outside a 
window, as compared with those on leaves kept at room temperature. 

Posey dried infected leaves of Ribes nigrum in the hot July sun for 
four hours. Urediniospores from these gave germination ranging 
from 3 to 45 per cent, with an average of 17 per cent. Spores from 
other leaves collected at the same time and dried inside in the shade 
gave 3 to 90 per cent germination, an average of 47 per cent. During 
dry, hot weather it has been found that the viability of the uredinio- 
spores out of doors is very low. 

Duff (30), in studying the factors affecting their viability, found 
that exposure to bright sunlight reduces their germination, the ultra- 
violet rays being the destructive agent. Their viability appears to 
him to be low. Three days after collection less than 50 per cent ger- 
minated in distilled water. In about two weeks germination was 
negligible even when stimulated by cooling to 2° to 5° C. 

Pennington 37 made germination tests of newly-matured uredinio- 
spores of Cronartium ribicola produced naturally in the vicinity of 
Lewis, N. Y., from early June until late in the autumn in hanging 
drops of tap water. During the first two weeks of June (frequent 
rain) the percentage of germination was high. From June 16 to 28 
no tests were made. On June 29 and July 1 (rather dry) only 1 per 
cent germinated. After this time, many tests were made with spores 
from various localities. By July 14 (very hot alternating with some 
rain) less than one-third of 1 per cent germinated, and from July 22 
to 26 (hot and dry) less than one-tenth of 1 per cent. Fresh yellow 
spores kept in an ice box gave no better results. On August 1 (rain 
July 29 and 30) 5 per cent germinated. After this (decidedly more 
rain) from 10 to 40 per cent germinated. The viability of these spores 
seemed to be greatly decreased by hot, dry weather and increased by 
cool rainy spells at the time they were produced. When the number 
of spores produced decreased because of hot, dry weather, their rate 
of germination also decreased and vice versa. 

Dor an (28) found that the limiting temperatures for the germina- 
tion of urediniospores are: Minimum 8°, optimum 14°, and maximum 
25° C. He calls attention to the fact that — 

There is apparently a relation between the season when spores occur and their tem- 
peratures for germination. The seciospores of Cronartium ribicola occur in the spring 
when the average temperature is lower than in the summer, the season of occurrence 
of the urediniospores of this fungus. The seciospores of this fungus have a niinimum 
temperature for germination, which is 3° C. lower than that of the urediniospores; an 
optimum 2° C. lower than that of the urediniospores; and a maximum 6° C. lower 
than that of the urediniospores. 



s? Pennington, L. H. Op. cit. 



60 BULLETIN 951, U. S. DEPARTMENT OE AGRICULTURE. 

Germination of fresh urediniospores usually takes place readily in 
tap water. Gravatt found that distilled water gave poorer germina- 
tion than tap water. Colley (20) found that the germ tube pushes 
through the exospore without the aid of a germ pore. The contents 
of the spore soon pass into the young germ tube, which may extend 
some distance over the surface of a Eibes leaf. Entry to the interior 
of the leaf appears to be through the stomata. The urediniospores 
germinate in about five and one-half hours (28) in drops of distilled 
water on glass slides. 

During germination studies in 1918, York 38 occasionally found 
germinating urediniospores which formed secondary conidia. An in- 
vestigation of the conditions causing their formation showed that 
newly formed urediniospores usually do not produce the secondary 
conidia in tap water, while old urediniospores were more likely to 
produce them. The following species produced them, the frequency 
increasing in the order named: Eibes lacustre, E. cynosbaii, E. vulgare, 
E. reclinatum, and E. nigrum. Urediniospores from E. glandulosum 
did not produce secondary conidia. Urediniospores exposed in bags 
of mosquito netting out of doors gave especially abundant secondary 
conidia. Urediniospores from E. nigrum produced secondary conidia 
in weak solutions of ammonia, maltose, tannic acid, gallic acid, malt 
extract plus gallic acid, lactose plus tannic acid, and lactose plus 
gallic acid. They were especially abundant in the last solution. 
They were not produced in pine decoction, weak solutions of ether, 
lactose, maltose plus tannic acid, and maltose plus gallic acid. A 
limited number were produced in water. They form on the ends of 
the germ tubes or laterally and are capable of producing a germ tube 
themselves. Similar secondary conidia have been noted by Tulasne 
(175) in cultures of Cronartium asclepiadeum, and they have been 
noted by Plowright (109) and Sappin-TroufTy (122) in other Uredi- 
nales. 

LONGEVITY OP THE UREDINIOSPORES. 

The first experiments in testing the longevity of urediniospores of 
Cronartium ribicola seem to have been carried out by McCubbin 39 in 
1916. His manuscript account of these experiments follows: 

The spores used for this series were all collected on the same day. They were dried 
on paper for a few hours and then placed in a number of small bottles plugged lightly 
with cotton, the contents of each bottle being available for a single inoculation. 
Half of these bottles were kept on a shelf in the laboratory, where they were dry and 
exposed to weak light, and the other half were placed under a bell jar on the soil 
in a garden, exposed to changes of humidity, temperature, and light. 

At stated intervals a bottle was taken from each set and the spores within were 
shaken up with a small amount of distilled water. By the use of a small atomizer 
the suspension of spores was then sprayed on the under side of the leaves of small 

3 8 York, H. H. Op. cit. 

39 McCubbin has very kindly allowed the use of his unpublished data so as to make this account as com- 
plete as possible. 



\ 



WHITE-PINE BLISTER RUST. 



\ 



61 



currant plants which had been previously set out in an isolated garden for the pur- 
pose. After inoculation the plants were covered for two days by a box having a glass 
lid. In all cases, water was sprinkled three or four times daily on the plants and 
on the inside of the box, to keep a high humidity. Unfortunately, during the whole 
of the period covered by this series of inoculation the weather was exceedingly hot 
and dry, and it was evident from a study of field conditions that infections could take 
place at this time only with the greatest difficulty. The adverse nature of the weather 
conditions will serve to explain the meager results. . . . 

The only positive result from this experiment was that the spores would retain 
their power of infection for a period of nine days at least; but so many failures oc- 
curred all through the course of the work that this period can not be regarded as 
establishing a maximum limit of life. 

It is interesting to note that the spores kept outside underwent a complete decolori- 
zation in two days, whereas those stored in the laboratory retained their normal 
color, with but little change throughout the whole time of the experiment. 

The inconclusive results obtained from the first set of inoculations in the field led 
to another later attempt with plants kept in the laboratory, for this purpose a number 
of small plants being used from which the leaves had been stripped, so as to induce 
the formation of secondary foliage. The methods employed in this case were the 
same as for the first set, exept that after inoculation the shoots were kept covered 
and moistened for the usual 48 hours inside large glass jars. (See Table IV.) 

Table IV. — Inoculation of currants with uredospores of different ages. 



Series. 


Age of 
spores 

(days). 


Date of inoculation, 
1916. 


Number 

of leaves 

used. 


Date of examina- 
tion, 1916. 


Result. 


I 


2 

4 

7 

8 

11 

13 

18 

24 

27 

31 




16 
16 
16 
14 
11 
16 
5 
7 
7 
7 


September 19 

September 22 

September 25 

September 26 

September 29 

October 1 

October 6 

October 12 

October 15 

October 19 


5 pustules on 2 leaves. 
67 pustules on 11 leaves. 


II 

Ill 


September 2 

September 5 

September 6 

September 9 

September 11 

September 16 

September 22 

September 25 

September 29 


IV 


10 pustules on 5 leaves. 
8 pustules on 2 leaves. 
2 pustules on 2 leaves. 


V 

VI 


VII 


VIII 

IX 


Do. 

Do. 


X 


Do. 







The results from these inoculations were somewhat better than those from the first 
set; infections were obtained from spores kept for 13 days after collection, as shown 
in the tabulated record. It is still thought that this period is far below the maximum 
period for which the spores will retain their vitality. 

In 1917 Gravatt and Taylor made a series of tests of urediniospores 
together with asciospores and teliospores. (See p. 38 for details 
of the experiment.) They were tested weekly beginning May 8. 
June 16 gave the last germination in lot A, while lot B persisted 
until July 2. Although germination persisted longer in lot B, it 
weakened decidedly somewhat earlier and was poorer practically 
throughout the test. (See Table II, p. 38.) 

In 1918, Duff (30) experimented on the longevity of urediniospores 
placed in a refrigerator at 2° to 5° C. two weeks after collection and 
tested in hanging drops of distilled water. He states that when 
placed in the refrigerator — 

A negligible percentage of spores were germinable, but reduction in temperature 
stimulated them to greatly increased germination. By this means a continually 



62 BULLETIN 057, U. S. DEPARTMENT OF AGRICULTURE. 

decreasing percentage of spores were kept in a viable condition, until after a lapse of 
a further period of about three weeks the number that germinated readily was negli- 
gible once more. Before the end of four weeks the spores had ceased to germinate. 

Extensive tests of the longevity of urediniospores were made by 
York, Overholts, and Taylor. 40 In one experiment leaves of Ribes 
nigrum, R. vulgare, and R. reclinatum were placed in bags of mosquito 
netting with the urediniospores outward. The bags were placed on 
three stakes at 6-inch intervals, the lowest one touching the ground 
and the highest 5 feet above the soil. The lowest spores remained 
viable only 6 to 9 days, while the upper ones were viable longest, 65 
days. A 2-day rain began the day after starting the experiment and 
again a 1-day rain two days later. The urediniospores from R. 
nigrum remained viable longest. Again infected leaves of Ribes 
nigrum, R. vulgare, R. cynosbati, and R. glandulosum were put in open 
boxes and exposed for 4 hours to the early morning sun. Viability 
persisted only 15 days. The urediniospores from Ribes nigrum re- 
mained viable longest. Urediniospores on pulled bushes of Ribes 
glandulosum and R. cynosbati hung in the bright sun remained viable 
only 4 days. Spores on leaves of Ribes nigrum dried in a plant press, 
then put in tight Mason jars and stored in an ice-box remained viable 
80 days. Successful inoculations were made with urediniospores col- 
lected 270 days previously and also with urediniospores from dead, 
overwintered leaves of the previous season. The age of the spores is 
not known, but they were certainly overwintered spores (180). It 
was found that viability in tap water persisted at least 169 days 
when the spore-bearing leaves were air dried and kept under slight 
pressure between sheets of heavy glazed paper. When kept out of 
doors but protected from rain, they retained viability for 100 days. 

In 1918 Pennington 41 made a number of tests of the longevity of 
urediniospores. In July and August urediniospores on Ribes leaves 
brought into the laboratory and air drie d lost their viability within 
a week when tested in drop cultures of tap water. On September 25 
many Ribes leaves were collected and allowed to dry between sheets 
of paper. The second day urediniospores from these leaves gave 
50 per cent germination when tested as above. The leaves were 
left in the dry air of the laboratory. The spores decreased*in viabil- 
ity until November 26 when but 1 per cent germinated. After that 
there was no germination. 

These results, showing a longevity ranging from 7 to 270 days 
under varying conditions, indicate the sensitiveness of the uredinio- 
spores to external factors. In addition it is quite possible that the 
mysiological condition of the host plant also has a profound effect 
upon these spores. 

40 York, H. H., O verholts, L. O., and Taylor, M. W. The longevity of the sporidia of Cornartium ribicola. 
Seen in manuscript. To be published in Phytopathology. 

41 Pennington, L. H. Op. cit. 



WHITE-PINE BLISTER EUST. 63 

THE TELIA AND TELIOSPORES. 

GENERATIONS OF TELIA. 

In 1918 Pennington 12 made observations upon the generations of 
telia at Lewis, N. Y. This was a season very favorable for the 
occurrence of distinct waves of spore production. The first genera- 
tion of telia appeared on June 28 with and following the second 
crop of uredinia. They were present throughout the rest of the 
season, but in the greatest abundance with and immediately follow- 
ing a new generation of uredinia. As compared with the uredinia, 
they were produced in relatively greater abundance with each 
succeeding generation. There were six distinct waves of telial 
production. 

SEASON OF PRODUCTION OF THE TELIA. 

The date when the first telia are produced varies from year to 
year with the earliness of the season. The earliest of which we have 
definite record is June 2, 1918, at North Conway, N. H. Table V 
(p. 72) gives data for the different regions of North America. 
The telia are formed until the Bibes leaves fall in the autumn. 
Drought is likely to cause premature shedding of diseased Bibes 
leaves soon after the first telia form. This greatly limits the produc- 
tion of new telia. 

DISTRIB LOTION OF THE TELIOSPORES. 

Because the teliospores are produced in more or less compact 
columellas they are normally not separated from the host plant. 
They do become distributed somewhat, however. Gravatt and 
Marshall (45) found that slugs eat telial columns from rusted Bibes 
leaves; also that sow bugs carry broken columns on their bodies. 
There seems to be no reason why insects and other animals may not 
do likewise. 

The telia are sometimes mechanically broken off and blown about 
by the wind. 

Diseased Bibes leaves fall to the ground and are blown about by 
the wind. Often they are broken into small pieces which may be 
blown long distances. In fact, York 43 found such bits of dead 
leaves in his spore traps 200 feet distant from the nearest Bibes 
bush. Telia on dead leaves kept out of doors in the shade are 
known to retain viability for 65 days, so that in this way the disease 
might appear in very unexpected places on pines at a greater dis- 
tance than the sporidia are carried in a viable condition. 



GERMINATION OF THE TELIOSPORES. 



The teliospores germinate readily in tap water and produce sporidia 
in 6 to 12 hours. 43 ' 44 Each spore produces normally a 4-celled pro- 



42 Pennington, L. H. Op. cit. 

« York, H. H. Op. cit. 

** York, H. H., Overholts, L. O., and Taylor, M. W. Op. cit. 



64 BULLETIN 057, U. S. DEPARTMENT OF AGRICULTURE. 

mycelium. Each cell regularly puts forth a stout sterigma on which 
the very thin walled, globular sporidium soon develops. ■ The 
sporidium has a tiny papillalike swelling where it was attached to the 
sterigma. The sporidia are 8 to 10 microns in diameter (20). The 
germ tubes of the teliospores, if developed under water, may not 
form promycelia but extend elongated hyphse (20) . Under favorable 
conditions a high percentage of the teliospores may germinate, but 
because of their aggregation into columellse it is impossible to make 
an exact count of the germinating spores. Cooling on ice stimulates 
viability markedly. 

LONGEVITY OP THE TELIOSPORES. 

The longevity of the teliospores of Oronartium ribicola does not 
seem to have received as much attention as that of the seciospores 
and urediniospores. Gravatt and Taylor made tests with telio- 
spores in 1917 similar to those described as made by them with 
seciospores and urediniospores. (See Table II.) Weekly tests 
showed that germination persisted in lot B 35 days, while it lasted 
56 days in lot A. Saprophytic fungi attacked the lot kept on the 
window sill, so that the test probably does not show the longevity 
of healthy teliospores. 

York 45 in 1918 found that teliospores were still capable of germi- 
nation in tap water after being kept on the plucked leaves 65 days 
out of doors in the shade. A similar test of teliospores kept in the 
dark in the laboratory gave germination for 90 days. 

THE SPORIDIA. 

SEASON OP PRODUCTION OF THE SPORIDIA. 

The sporidia may be produced as soon as the telium is mature, if 
there is sufficient moisture in the air for a number of hours. 

The telia may remain alive on dry dead leaves out of doors for 
more than 65 days, so that sporidia might be produced well into the 
whiter in mild seasons, thus prolonging the danger season for pines. 

DISTANCE OF DISSEMINATION OF THE SPORIDIA. 

In work with spore traps by Pennington 46 and Snell in 1918, 
sporidia were caught up to 60 feet from very heavily infected Kibes 
bushes. This was in the eastern Adirondacks, about 8 miles from 
Lake Champlain. Hundreds of pines were examined for infections. 
In no case was infection found on pines as far as 200 feet from Bibes 
plants. Pennington made a study of nine outbreaks in pines in the 
Adirondacks. The infection on pines was confined to an area 
within 100 to 200 feet of the Kibes plants which infected the pines. 

« York, H. H. Op. cit. 

« Pennington, L. H. Op. cit. 



WHITE-PINE BLISTER EUST. 65 

In 1919, Pennington (cited in Spaulding, 146) caught sporidia up 
to 294 feet distant, but they failed to germinate. Under favorable 
conditions, sporidia caught at a distance of 177 feet germinated, 
but none beyond this distance. 

York, 47 working in the White Mountain region of New Hampshire, 
in 1918, found that sporidia were quite common in spore traps 
exposed 24 hours at a distance of 200 feet from the diseased Ribes 
bush. York (cited in Spaulding, 146) in 1919 caught sporidia, under 
favorable conditions, at 600 feet distance, which germinated. 

The infection of pines is said by McCubbin (88) to depend on 
" (1) The nearness of cultivated Ribes, particularly black currants; 
(2) the number of wild Ribes present; (3) the moistness of the situ- 
ation." York 47 concluded that these factors are "topographical 
features, direction of the wind when sporidia are produced, humidity 
of the air, precipitation, and the nature and density of vetegation 
between the Ribes and pines." Pennington 48 stated that weather 
conditions have much to do with the degree of infection that occurs 
on pines; cool, moist situations favor infection; intervening barriers 
of vegetation tend to limit infection; the amount of infection under 
given conditions varies directly as the extent of Ribes leaf surface 
and inversely as the square of the distance from Ribes. The writer 
(145) said the width of the Ribes-free zone around pines is largely to 
be governed by topographical features; direction of the wind pre- 
vailing at the time the sporidia are produced; humidity; age of the 
pines; exposure and species of Ribes; and the composition, height, 
and density of the vegetation between the Ribes plants and the pines. 
The experiments with the sporidia show that high humidity is neces- 
sary for these spores to live any length of time. It alone may very 
largely determine whether infection can take place. 

A few specific instances show the effect of these factors in actual 
outbreaks. On July 10, 1917, on Gerrish Island, at Kittery Point, 
Me., Gravatt investigated the small trees of Pinus strobus within a 
radius of 15 feet of a bush of Ribes Mrtellum to determine the spread 
of infection. The gooseberry was a small bush, having approximately 
270 small leaves and there were no other Ribes near by to influence 
the result on pines. The ages of the pines were as follows: Two 
years, 12; 3 years, 17; and 5 years, 82; a total of 128 pines, none 
over 5 years of age. There were 77 separate infections on 54 dis- 
eased trees, 44 of these infections being on 2-year wood. As the 
oldest pines were 5 years old and most of the infections which occurred 
the year before were probably not detected, this infection of more 
than 40 per cent resulted from an exposure of only a little more than 

« York, H. H. Op. cit. 

48 Pennington, L. H. Op. cit. 

46103°— 21— Bull. 957 5 



66 BULLETIN 957, U. S. DEPAKTMENT OE AGEICULTURE. 

3 years to the disease. To judge from the results of the first five 
years, it is not likely that any of the pines within the 15-foot radius 
would remain alive at the end of 12 or 15 years. Pines outside the 
15-foot radius from the bush showed only scattering infections. 
This was in a location well protected from strong winds. 

In another case where Ribes nigrum was well exposed to the strong 
storm wind from the neighboring White Mountains, York found 
pine infections up to 600 yeards distant from heavily diseased Ribes 
bushes. 49 

In the outbreak at Kittery Point, Me., Posey found that a num- 
ber of Ribes nigrum bushes so located that the wind had moderate 
access to them caused infection of Pinus strobus trees up to a distance 
of about 300 yards. 

Our studies (146) of the distance of distribution of the various spore forms and of 
the distance that infection has actually occurred upon pines from known infected 
Ribes indicate that the Ribes-free zone should be, under average conditions, 200 to 
300 yards in width. It should be much more where conditions are exceptionally 
favorable for transfer of the spores from Ribes to pine, i. e., near large bodies of Ribes, 
where there is no screen of vegetation over the Ribes or between the Ribes and the 
pines, or in exceptionally humid situations. The cultivated black currant (Ribes 
nigrum) should not be allowed in an infected pine district because of the special 
danger from it. 

Studies by York 49 of the natural infections of pines show that the 
sporidia are blown along roads cut through heavy forest cover and 
that they do not reach pines located in isolated small pockets in the 
dense forest. Trapping of sporidia from Ribes located under dense 
cover of black alder yielded sporidia only up to a distance of 75 feet. 
Traps set 20 feet in the air and well above the cover, but directly 
over the Ribes bushes, caught no sporidia. 

AGENTS DISSEMINATING THE SPORIDIA. 

It is apparent that the sporidia produced by the teliospores of 
Cronartium ribicola are largely disseminated by the wind. Observa- 
tions in various areas where white pines have become infected from 
neighboring Ribes bushes show plainly that this is the case. In such 
cases the infection is most intense nearest the Ribes bush acting as a 
center of infection. The degree of infection decreases as the distance 
from the center increases. Other conditions being equal, the distance 
of pine infections from the infection center is very short where there 
is a thick screen over and around that center, while the converse is 
true where the Ribes infection center is well out in the open. (See 
pp. 64 to 66 for data bearing on this matter.) 

Minor disseminating agents are known, and their number will 
undoubtedly be increased by future investigations. The investi- 
gations of Gravatt and Marshall (45) in the experimental greenhouse 

«York, H. H. Op. eit. 



WHITE-PINE BLISTER RUST. 67 

at Washington, D. C, showed that weevils, snails, slugs, and sow 
bugs feed on the telia. The voided teliospores retained viability in 
a few instances. This indicated that similar animals might be active 
agents in the local distribution of these spores out of doors. Investi- 
gations by Snell (127) in 1918 at Lewis, N. Y., showed that a number 
of different types of insects feed on rusted leaves of Kibes bushes and 
may serve as carriers of the sporidia directly from plant to plant, or 
indirectly b}^ the voided teliospores. Marshall, in 1917, found that 
the moist sporidia allowed to dry on a feather are not easily dis- 
lodged therefrom, either by wind or by brushing of the feather on 
cloth. This suggests the possible carriage of sporidia by migrating 
birds in the fall for long distances. Their very short life, as deter- 
mined by York (see pp. 67-68), however, probably prevents their 
causing infection of pines under these conditions. 

The remarks on the carriage of seciospores by currents of air gen- 
erated by fast-moving automobiles, steam trains, and electric cars also 
apply to the sporidia. (See p. 36.) 

GERMINATION OF THE SPORIDIA. 

Gravatt, Colley (20), and York, Overholts, and Taylor 50 found 
that the sporidia germinate immediately in tap water under favor- 
able conditions. They germinate like ordinary fungus conidia, by 
pushing forth a germ tube which is relatively large. They are capable 
of germination as soon as they reach full size, even though still 
attached to the promycelium. The germ tube normally develops 
until a mycelium is formed. In some cases the germ tube soon forms 
a secondary sporidium which in turn may germinate. The viability 
of the fresh sporidia is high, as many as 90 per cent germinating 
within 24 hours. 

LONGEVITY OF THE SPORIDIA. 

The sporidia of Cronartium ribicola are so thin walled and fragile 
in character that it seems self-evident that they are short-lived spores. 
This supposition has been proved to be correct by the work of York 
and Overholts in the summer and autumn of 1918 and of York and 
Taylor in 1919 (cited in Spaulding, 146). Colley (20) found in 1917 
that fresh sporidia germinated readily in distilled-water cultures. 
York, Overholts, and Taylor 50 dried the sporidia on glass slides and 
tested their viability after varying intervals. Very slight germina- 
tion resulted after 10 minutes exposure by an open window at 66° F. 
when light rain was falling. None survived when exposed to bright 
sunlight for 10 minutes with a temperature of 77°F. Nor did they 
survive when pieces of Bibes leaves bearing the telia and sporidia 
were exposed to sunlight for 10 minutes at 85° F. and with a humidity 

» York, H. H., Overholts, L. O., and Taylor, M. W. Op. cit. 



68 BULLETIN 95^7, U. S. DEPARTMENT OF AGRICULTURE. 

of 30.5 per cent. In another experiment the sporidia were placed 
on the periderm of white-pine twigs of the same season's growth and 
on living leaves of Pinus strobus and P. rigida. They were then 
exposed dry at 66° F. and with a humidity of 90 per cent. None 
survived for 10 minutes. At 72° F. and a humidity of 69 per cent 
none survived for 10 minutes. They conclude that the sporidia can 
endure very little desiccation and are short lived under seemingly 
optimum conditions. Abundant moisture is necessary for infection 
of pines to occur. 

HETEROSCISM OF THE SPORIDIA. 

A number of tests have been made to learn whether Bibes might 
become infected by sporidia of Cronartium ribicola. Jaczewski (59) 
states that experiments have shown that they will not infect Ribes 
leaves. In 1913, Clinton, Stewart, and the writer (151) inoculated 
Ribes nigrum leaves with teliospores overwintered out of doors, but 
there were no infections. In 1912 the writer tested fresh teliospores 
without infection occurring (136). In 1917 Gravatt made several 
tests of fresh, sporidia-producing telia, but no infection resulted. 

OVERWINTERING OF CRONARTIUM RIBICOLA. 
Overwintering on Pines. 

The generally accepted view has been that Cronartium ribicola 
lives over winter by means of the mycelium in the bark of living 
infected pines and by this means only (142). A number of writers 
have mentioned cases where their observations seemed to indicate 
the possible overwintering on infected Ribes, but nothing that could 
be accepted as real evidence was offered until the last few years. 
There is no question that the fungus overwinters chiefly in the 
infected living pine trees and has been carried in the dormant con- 
dition from continent to continent in young infected pines. 

It has been discovered, as has been mentioned earlier, that Cro- 
nartium ribicola may overwinter as mycelium in infected branches 
cut from diseased trees late in the fall, or during the winter, and 
allowed to lie until spring. Then, if these cut-off branches lie close 
to damp soil or with the cut ends in a stream or pool, fresh vigorous 
secia are produced (89). Still another phase of overwintering was 
discovered by Dosdall (29) in Minnesota. On April 19, 1918, a 
dead branch of white pine, bearing an infection which bore secia in 
1917, was collected. Germination tests in distilled water showed 
that 1 to 2 per cent of the old seciospores were still viable. 

Overwintering on Ribes Plants. 

Investigations of overwintering of Cronartium ribicola on Ribes 
plants, in Europe, seem to be limited to field observations. They 



WHITE-PINE BLISTER RUST. 69 

relate to instances where diseased Ribes bushes were found widely 
separated from Pinus strobus or from all 5-leaved pines (6). These 
necessarily depend for reliability upon the observer's complete and 
minute knowledge of the Ribes and pines within considerable areas. 
Hence, such observations are of very uncertain value. Investiga- 
tions showing that the seciospores of this fungus are distributed for 
miles largely invalidate such observations so far as overwintering is 
concerned. 

In North America, investigations of overwintering on Ribes plants 
have been along the following lines: (1) By means of spores adhering 
to dormant Ribes plants, (2) in dormant or partially opened Ribes 
buds, (3) in living Ribes leaves which themselves lived over winter, 
(4) on dead Ribes leaves, and (5) on infected Ribes stems. 

Overwintering by means of spores adhering to dormant Ribes 
plants has been investigated in several ways. A great many field 
observations have been made upon bushes diseased heavily one year 
and not infected the succeeding year. Cases where bushes were 
shipped from known diseased localities and have shown the rust the 
next season in their new locations, have been considered, but the 
evidence has been too incomplete to be seriously considered except 
as it might help to confirm or refute other stronger evidence. A 
great many Ribes plants have been used by the writer in greenhouse 
experiments ; they shed their leaves and become dormant for several 
months, yet there has been no hint of the carrying over of the fungus 
upon them from one season to the next. A cooperative experiment 
was made with Stewart (151), using 500 plants of Ribes nigrum, which 
in the summer of 1912 were heavily infected. The leaves dropped 
normally. They were then dug and most of them heeled in out of 
doors until February, 1913. They were then brought into green- 
houses in six widely separated localities and allowed to put forth 
new leaves. Examination of some of these dormant plants by Arthur 
and Retry showed that plenty of urediniospores still adhered to the 
stems and buds. Inoculations with these spores did not give any 
infection, so that they presumably had lost their viability. The 
results reported by six different investigators showed no infection 
appearing on the new leaves. 

Howitt and McCubbin (56) in attempting to solve the overwinter- 
ing problem, made the following tests : 

(1) In the fall of 1914, 16 black and 7 red currant bushes and 1 gooseberry bush, 
all badly rusted, were stripped of leaves and placed in cold storage, where they 
remained until March 16, 1915. At this date they were removed and planted in a 
greenhouse. All grew well and produced healthy leaves and fruit and were entirely 
free from rust throughout the summer. In addition, 17 black currant bushes, which 
had been badly rusted in 1914 and which were wintered in the field, were added to 
the above on April 21, 1915. These also grew normally and without rust. 



70 BULLETIN" 957, IT. S. DEPARTMENT OF AGRICULTURE. 

(2) Seventy-eight black currant bushes, badly rusted in 1914 were wintered in 
the nursery rows, and transplanted April 12, in various gardens, isolated as far as 
possible from infected white pines and currants. These were inspected six times 
during the summer, the last inspection being made on October 2. At this date all 
were still free from rust except two bushes, on each of which a few rusted leaves 
were found. There is reason, however, to suspect that these infections might have 
been due to spores carried from currants about a mile distant from the garden in 
which they occurred. In no case was rust found on any of these currants which 
were located more than a mile from a source of infection. 

(3) A number of bushes from the same source as No. 2 were planted iu five lots in 
a region known from personal observation to have been entirely free from the rust in 
1914, and which is 60 miles from the nearest known source of infection. Of the 100 
bushes set out here only one developed rust, and this late in the season. All con- 
ceivable sources for this infection have been accounted for except two, viz, the 
wintering over of the rust on the currant itself, or accidental infection from spores 
carried on the writer's clothing while making an inspection on May 24. 

In 1917, V. B. Stewart (152) tested the possible overwintering of 
the fungus by means of spores adhering to diseased bushes of Ribes 
nigrum. These were heavily infected in 1915 and 1916. In August, 
1916, they were defoliated, and 200 were dug and placed in a storage 
cellar in October, where they remained all winter. In 1917, they 
were sent to Ithaca, N. Y., and set out in a field. The disease had 
not been known within 40 miles. The disease did not appear upon 
them up to October 9, 1917. 

The possibility of overwintering in Ribes buds was brought to the 
writer's attention by infections of petioles (131, 134, 135), by which 
means it seemed entirely possible for the mycelium to travel from a 
leaf blade down the petiole and thence into the stem and bud in the 
axil of the leaf. While many diseased petioles have been examined, 
no indication of the migration of the hyphse into the stem or bud has 
yet been seen. Direct examination of buds on heavily infected 
bushes has also failed to yield any indication of bud-scale infection 
(151). McCubbin (85) suggested but could not prove that infection 
of partially opened buds late in the fall might result in some of the 
infected leaflets surviving the winter and developing the disease the 
next spring. York 51 successfully inoculated the inner bud scales of 
opening buds of Ribes nigrum with asciospores, suggesting overwinter- 
ing in this way. 

The possibility of green leaves living over winter on Ribes plants 
out of dc irs has been investigated. In three cases, the writer had 
Ribes plants growing in pots plunged in sand out of doors at Wash- 
ington, D. C, retain green leaves through the winter until the spring 
weather of March, 1918, set in. One plant of Cumberland goose- 
berry and two plants of Utah Yellow currants did this. They were 
taken as specimens on March 22, when warmer weather set in. The 

5 1 York, H. H. Op. cit. 



WHITE-PINE BLISTER RUST. 71 

previous winter a single seedling plant of an unknown species bore 
leaves flat on the soil under similar conditions until March 12, 1917, 
when it was brought into the greenhouse and inoculated. It 
promptly took the disease on the overwintered leaves.' York 52 found 
Ribes glandulosum plants in the spring of 1918 which bore overwin- 
tered leaves that later became infected naturally. In such cases, it 
would be easy to understand that late infections in the fall might lie 
dormant until spring and then produce vigorous uredinia. More 
time is necessary to determine whether this actually occurs. 

As stated previously, infection of petioles by Cronartium ribicola 
is quite common. Early in 1917 Colley (17) discovered that infected 
petioles often had telia and masses of active mycelium as well as 
uredinia in the central pith. This raised the question of the possi- 
bility of such mycelium remaining active until spring and producing 
new uredinia. 

Whether the fungus can live over winter on dead diseased leaves 
seemed unlikely in view of the negative results of Arthur and Petry 
(151) with urediniospores from stems of plants diseased the preceding 
summer, and the negative results of Stewart (151) with material 
overwintered out of doors at Geneva, N. Y. Howitt and McCubbin 
(56) early in 1915 attempted to produce infection by spores which 
remained over winter out of doors on dead Ribes leaves. All of their 
attempts were unsuccessful. In the spring of 1918 York (180) and 
the writer obtained infections with urediniospores overwintered out 
of doors in Massachusetts on dead Ribes leaves, proving that uredi- 
niospores may survive the winter. This was repeated in the spring 
of 1919 by Taylor (157). 

The possibility of infection on Ribes stems was early investigated 
by the writer but with no success. Many inoculations were made on 
young Ribes shoots by the writer and later by Gravatt, Doran (28), 
and York 53 but without success. However, in the summer of 1917, 
Posey and Gravatt (112) discovered fruiting uredinia on the young 
shoots of Ribes Mrtellum at Kittery Point, Me. They inoculated 
other young shoots with seciospores and secured mature uredinia. 
Colley found uredinia in the pith of these infected stems. Gravatt 
later inoculated young seedlings of Ribes fasciculatum in the green- 
house with aeciospores and secured heavy infection of the cotyledons. 
In one seedling the fungus also attacked the stem just below the 
diseased cotyledons and developed several uredinia (PL V, fig. 2). 
Later, however, the plant outgrew the disease. Taylor and York 
have successfully inoculated stems of several species of Ribes. (See 
p. 50.) 

That Cronartium ribicola overwinters on Ribes is established. 

52 York, H. H. Op. cit. 



72 



BULLETIN 95*7, TJ. S. DEPARTMENT OF AGRICULTURE. 



IMPORTANT DATES IN THE LIFE HISTORY OF CRONARTIUM RIBICOLA. 

Table V shows some of the more important dates in the develop- 
ment of the white-pine blister rust and their variation according to 
locality. These dates are rarely the earliest or latest possible ones, 
but are based upon notes actually made respecting the point covered. 
It is hoped to extend this table greatly and approach nearer the 
actual date when each stage of development is reached by the fungus 
in the various regions. Southern New England is here made to 
include Massachusetts, Connecticut, and Rhode Island. Northern 
New York is understood to include approximately that part of the 
State lying north of a line between Glens Falls and Oswego. The 
Lake States include Michigan, Wisconsin, and Minnesota. Similar 
data for Europe are given in the last columns of the table for the 
sake of comparison. 



Table V. — Important dates in the life history of Cronartium ribicola, as observed in 

America and in Europe. a 

[To economize space, the century digits 19 are omitted in noting the year of each observation; thus 

'09=1909.] 



Development 
noted. 



First closed blis-l 
ters 1 



First open blis- 
ters 



Last yellow blis- 
ters 



First uredinia . . . 



First telia. 



First pycnial ( 
drops \ 



United States. 



Southern 

New 
England. 



Apr. 7/17 
Apr. 5/18 



June 22, 
May 22, 
May 16, 
June 3, 
May 25, 
June 2, 
May 3, 
Apr. 17, 
Apr. 7, 
Mar. 28, 



June 3, '16 
June 13, '17 
May 29, '18 



June 17, '16 
July 21, '17 
June 19, '18 
July 30, '19 



June 21, '16 
June 7, '18 



Southern 
New York, 
New Jersey, 
and eastern 

Pennsyl- 
vania. 



Apr. 30, '11 
Apr. 28, '17 
May 4, '18 



June 1,'09 
July 27, '10 
May 5, '11 
June 10, '13 
May 20/15 
Apr. 7/16 
Apr. 28/17 
May 16/18 



June 10, '13 
July 18/16 



Northern 

New 
England. 



Apr. 15/10 

Apr. 7, '17 
May 13/18 



June 14, '09 
May 26/10 
June 9/11 
May 15/12 
June 6/13 
May 15/15 
May 15/16 
Apr. 29, '17 
Apr. 23/18 
Apr. 7/19 

June 7/10 
July 1, '17 
July 21/18 



July 24/13 
July 14/14 
May 20/17 
May 16/18 
May 23/19 

June 26, '13 
July 24/16 
June 10, '17 
June 2/18 
June 6/19 

July 26/13 
June 7/18 



Northern 
New Y"ork. 



Apr. 26/18 
Apr. 21/19 



June 8/09 
May 12/10 
Apr. 26, '18 
Apr. 23/19 



June 12/18 
June 26/19 



July 25/09 
July 18/16 
June 28/17 
May 16/18 
May 23/19 

Aug. 23/17 
June 26/18 
June 30, '19 



July 3/18 
June 26/19 



Lake States, 



May 5/17 
Apr. 19/18 
Apr. 18, '19 



May 24/16 
May 8/17 
Apr. 19/18 
Apr. 22/19 



June 24, '17 
June 24/19 
Sept. 16, '19c 



June 9/16 
June 12/17 
May 18/18 
June 5/19 



July 21/16 
June 12/17 
July 8/18 
July 10/19 



Europe. 



Date. 



Apr. 16, '90 
Apr. 15, '01 
Apr. 7/09 
Mar. 17/18 

►Apr. 15/90 

Apr. 29, '96 
Apr. 28/08 



May 15 

May 9 

June 1 

May 19/18 



May 23 

May 30 

May 26 

June 2/19 



Aug. 1. 
ljuly 30 



June. 



Local- 
ity.6 



H 
M 
D 

F 

H 

Bo 

S 

B 



H 

D 

Bo 

F 

D 
B 
B 

S 



a Notations for Ontario, Canada: First open blisters — May 10, 1917, and June 22, 1918; first uredinia — 
June 24, 1915, and July 20, 1916; first telia— June 24, 1915. 

& Abbreviations used: B= Berlin, Bo= Bohemia, D= Denmark, F= France, H= Hamburg, M= Mu- 
nich, S= Silesia. 

c This date is an extraordinarily late one for sscia to be formed, but it is included here to show the possi- 
bility of the serial season being prolonged throughout the summer. 



WHITE-PINE BLISTER RUST. 73 

CONTROL OF THE WHITE-PINE BLISTER RUST. 
Significant Factors Which Determine Control. 

FACTORS IN THE FUNGUS. 

The significant features in the life history of Cronartium ribicola 
are as follows: The pycnospores are apparently functionless; the 
seciospores are not known to infect pines, but they do infect Bibes 
readily; the urediniospores are not known to infect pines, but they 
do infect Ribes; the sporidia produced by the teliospores are not 
known to infect Ribes, but they do infect pines. 

The spores are all distributed by the wind much more than by any 
other agency. The seciospores are carried and are capable of infect- 
ing Ribes leaves miles away from their source. The urediniospores 
are distributed a number of hundred yards, but appear to lose their 
viability soon, so that infection by them is rather limited in extent. 
The sporidia produced by the teliospores appear to be distributed 
to a distance of a few hundred yards, but they are so frail that they 
soon lose viability. Infection by them is limited to 100 to 600 yards 
as a general thing, and more commonly the former than the latter 
distance. 

The fungus lives over winter most commonly by means of the 
mycelium, presumably in the needles and certainly in the bark of 
infected white pines. It occasionally overwinters by means of the 
seciospores in cankers of pine bark or by the urediniospores on Ribes 
leaves. The seciospores produced by the overwintered mycelium 
in the pine bark are the principal source of infection of the Ribes 
leaves each spring. The seciospores carry the disease far and wide 
for miles to the new Ribes leaves. The urediniospores intensify 
the disease in the vicinity where it is started by the seciospores. 
The sporidia carry the disease back to those pines which are rela- 
tively near infected Ribes bushes. 

High humidity of the air is necessary for any of the spore forms to 
germinate and to produce infection. 

FACTORS IN THE ENVIRONMENT. 

CLIMATIC FACTORS. 

Climate may be reduced to the three most potent factors — mois- 
ture, sunshine, and wind. Cronartium ribicola is absolutely de- 
pendent upon abundant moisture for its development. Drought, 
especially if prolonged, apparently may hinder the development of 
the secia (49, 135). Lack of moisture prevents germination of all 
the different forms of spores. It prevents or very greatly reduces 
the extent of infection on Ribes plants by seciospores. It prevents 
the production of new generations of urediniospores, 53 and conse- 
quently prevents the abundant formation of uredinia as well as 

53 Pennington, L. H. Op. cit. 



74 BULLETIN 957, XT. S. DEPARTMENT OF AGRICULTURE. 

restricts the spread of this stage of the fungus. Moreover, drought 
very largely reduces viability of the urediniospores. 54 With the 
very short-lived sporidia of the teliospores it is evident that lack of 
moisture immediately after their production may entirely prevent 
their infecting pines at all, and drought is known greatly to limit 
their formation. Drought causes the premature fall of leaves of 
Ribes bushes so as to leave practically nothing for the fungus to sub- 
sist upon late in the season. Thus the crop of teliospores is so 
greatly reduced in times of drought that infection of pines is largely 
or entirely prevented. Drought kills many young Ribes seedlings 
and many are winterkilled (23, p. 8). On the other hand, rain 
undoubtedly beats down the spores floating in the air and washes 
spores from the host plants, so that infection by them is prevented. 

Sunshine, by influencing the moisture of the air, may be very potent 
in reducing the activities of the fungus. It has a direct deleterious 
effect upon the spores 54 (30, 88) . It is an open question whether the 
erratic germination of the urediniospores is not due to this action of 
the sun's rays. By promoting the quick maturity and hardening of 
the leaves of Ribes in the open, bright sunlight may greatly reduce 
the infection which develops upon them. 

Wind is apparently the chief agent disseminating all forms of 
spores of this fungus. Its activity greatly influences the spread of 
the disease. 

THE AGENCY OF MAN. 

Man is a most potent agent in the dissemination of the white-pine 
blister rust. Through his activities it has made all of its known 
long-distance jumps. There is reason to believe that it is a native 
of northern Asia, whence it spread to Europe. The extensive trade 
in young trees of Pinus strobus is known to have been the means of 
introduction of this disease to many parts of Europe (111, 120, 155, 
162, 170). It certainly came to North America in young white-pine 
stock from Europe and has attained its present wide distribution 
here in such imported stock. See figures 2 to 12, showing the progress 
of the disease since 1909. 

INSECTS AND OTHER ANIMAL FACTORS. 

Various animals (insects, snails, mammals, man, etc.) may aid in 
the distribution of the disease by carrying spores on their bodies, or 
they may retard or reduce the fruiting of the fungus by eating the sori 
on both pines and Ribes; and others such as gipsy-moth larv«3, other 
insects, snails, and squirrels may even eat the surrounding bark on 
pines, so that no more sori can form. (PL III.) In 1918, Penning- 
ton 54 estimated that the production of aeciospores in the Adirondacks 

M Pennington, L. H. Op. cit. 



WHITE-PINE BLISTER RUST. 75 

was reduced about 15 per cent by the eating of infected bark by mice, 
squirrels, porcupines, etc. Posey and Gravatt 55 found that squirrels 
had eaten 17 per cent of the 93cia-bearing bark in a given area at 
Kittery Point, Me., and this is substantially true for the infected 
forests of that section. The leaf-eating insects and mammals may 
so reduce the leafage of Bibes plants as to reduce the disease materially 
in a given locality. 

OTHER FUNGOUS FACTORS. 

Other fungi are of some importance also. At Kittery Point, Me., 
Colley (20) and Posey and Gravatt 55 found that secondary fungi 
work in the pine bark infected by Cronartium ribicola in such a way 
as nearty or entirely to kill out the latter, probably by killing the 
bark around the cankers so that the blister rust is starved out. This 
sort of thing is quite general where white pines are generally infected 
by Cronartium ribicola. Very often it appears that the diseased 
pines are killed finally by the secondary fungi rather than by the 
blister rust. The secia of Cronartium ribicola are sometimes attacked 
directly by other fungi (80, 168, 172). It has also been found that 
the uredinia and telia are attacked by various fungi, so that their 
efficiency is greatly reduced locally (116). Fungi parasitic upon the 
leaves of Ribes sp., causing their premature fall, may greatly reduce 
the leafage available for the blister-rust fungus to attack and thus 
reduce the quantity of teliospores to produce infection on pines. 

FACTORS IN THE HOSTS. 

There are certain factors in the hosts themselves which are impor- 
tant in the control of this disease where it has once become estab- 
lished. These are resistance by some of the hosts to the disease and 
the natural suppression of the lower branches of white pines. 

Among the white pines the blister rust attacks Pinus strobus with 
especial virulence. It does not attack P. cembra nearly so readily. 
Experience shows that P. flexilis 53 is decidedly susceptible to 
it. This is confirmed by Moir's studies in Sweden. Knowledge of 
the relative susceptibility of the pines is extremely limited, because 
the disease has been in North America too short a time and has not 
yet reached any but the eastern white pine. In Europe, where the 
older outbreaks have occurred, there undoubtedly is an opportunity 
to obtain definite data on the relative susceptibility of the pines. 
It may prove feasible ultimately to plant another species of white 
pine which is not nearly so susceptible to the blister rust and which 
also is of value as a timber tree. 



55 Posey, G. B., and Gravatt, G. F. Field studies on the white-pine blister rust at Kittery Point, Me. 
Seen in manuscript. 

56 Pennington, L. H., Snell, W. H., York, H. H., and Spaulding, P. Investigations of Cronartium 
ribicola in 1920. Seen in manuscript. Published in Phytopathology, v. 11, p. 170-172. 1921. 



76 BULLETIN 957, U. S. DEPARTMENT OF AGRICULTURE. 

It has been possible to learn a little more concerning resistant 
species and varieties of Ribes. Ribes alpinum is found to be immune 
in America, although it is stated that it takes the blister rust in 
Europe. There does not appear to be any resistant species which 
will take the place of the cultivated R. nigrum (the black currant), 
or of R. odoratum (aureum) (the flowering currant). Among the 
cultivated red currants the varieties Franco-German, London, 
Rivers, and Holland have shown themselves very resistant. In 
generally infected areas these may prove of value to replace the 
more susceptible varieties. 

In the outbreak area at Kittery Point, Me., one of the oldest in 
North America, the infected pines are thickly crowded together and 
mostly range in height from 15 feet upward. The lower branches 
are being suppressed and are dying rapidly from overcrowding. 
Experience has shown that trees and branches attacked by the 
blister rust are weaker than healthy ones and are more apt to die 
from drought and suppression. Posey and Gravatt 57 find that this 
natural suppression of lower branches at Kittery Point has resulted 
in the killing of many entire branches bearing blister-rust cankers 
well out from the trunk of the tree. In such cases the disease in the 
dead branches is killed also. They find that about 15 per cent of the 
trees originally infected have thus recovered from the disease before 
it reached their trunks. As above intimated, this process is probably 
at its height in this area, since suppression of the branches is ap- 
parently at its maximum. 

Experiments in Control in Europe. 

In experimenting with the white-pine blister rust, the European 
investigator has always had a different viewpoint from that of the 
investigator in North America. This has been due to two reasons — 
the disease was possibly native in Europe, certainly in Asia, but was 
introduced into North America; Pinus strobus, the favorite pine host 
for the fungus, is native in North America and introduced into 
Europe. That is, the situation is exactly reversed in every respect 
in North America as compared with Europe. 

The disease is generally considered to have been native in the 
Alps and in the Ural Mountains upon Pinus cernbra. It appeared 
in widely separated localities through Northern Europe before plant 
pathology had developed to any extent. That is, organized quaran- 
tines, present methods of spraying, and many other methods now 
used in fighting plant diseases were unknown at that time. The 
fact that the disease was prevalent practically throughout northern 
Europe before it became generally known, showed plainly that it 
was firmly established throughout that region. This meant that 

w Posey, G. B., and Gravatt, G. F. Op. cit. 



WHITE-PINE BLISTER RUST. 77 

eradication was impossible. Local control has therefore been the 
only aim of the Europeans. Besides all this, the application of 
methods of control to plant diseases in Europe has never been devel- 
oped to such a point as it has in North America except for relatively 
few diseases of the more important cultivated crops. There has 
apparently never been a well-planned investigation of the control 
of this disease extending over a number of years anywhere in Europe. 
All European publications upon control are fragmentary. It is 
evident that many scattered efforts have been made to control the 
disease there, but the results have never been published. 

As stated above, the status of Pinus strobus in Europe is entirely 
different from its status in North America. While it has been more 
than 200 years since it was introduced into Europe (5) , it of course 
has not approached the distribution that a tree does in its native 
region. It has been widely distributed in Europe as a park and orna- 
mental tree and has been very popular for this purpose. As a forest 
tree it is a species which is planted in relatively small blocks and even 
then only on an experimental scale. In Europe it is essentially an 
ornamental tree rather than an important timber tree. Its total 
value there is exceedingly small compared to its total value in North 
America. 

Legislation against plant diseases in Europe is so complicated that 
no attempt will be made here to give an outline of it. Incidentally, 
it should be stated that Tubeuf (162, 163, 164, 169, 170, 174) has 
repeatedly called attention to the fact that commercial nurseries 
have been and are still spreading this disease throughout Germany. 
In 1904 he (170) repeats earlier demands for a national control of 
the forest-tree nursery trade and goes so far as to refute the state- 
ments of Schwarz (125) that this disease in the nurseries at Halsten- 
bek is absent or negligible. It is evident that the nursery trade domi- 
nated the situation and prevented such action. 

Since the disease on Kibes plants is essentially one of the leaves, 
there has been an apparent chance for success by spraying them. 
Tubeuf (165) seems to have been the first to report on such tests. 
He sprayed Bibes leaves in the greenhouse with Bordeaux mixture 
and then set the sprayed plants among those already diseased. 
Numerous uredinia soon developed on the lower sides of the sprayed 
leaves. Jaczewski (58) says that spraying with Bordeaux mixture 
is not very effective. 

Ewert (37) in 1912, to prove whether infection of Bibes leaves 
always occurs on the lower side only, made a test on a bush of Bibes 
nigrum. This bush was one of a number of Ribes plants upon which 
Cronartium ribicola had appeared every year for a decade. One-half 
of the bush was sprayed on the lower sides of the leaves only; the 
other half was untreated. Spraying was done on March 28, April 9 



78 BULLETIN 957, U. S. DEPARTMENT OF AGRICULTURE. 

and 27, May 3, 7, and 20, June 1, and July 9. About 1,000 leaves were 
borne on the bush on June 27 so that each half had approximately 
500 leaves. At that time the unsprayed half had about 250 leaves 
so heavily infected on the lower surface that some were already 
about to fall. On the sprayed half only 10 leaves were infected, all 
but one of these had only 1 or 2 sori, a single leaf had more. In this 
season the fungus attacked all the Bibes bushes very heavily, more 
so than for 10 years preceding. 

It is noted that Ewert does not say specifically that sori formed only 
on the lower surface of the leaves. They may be presumed to have 
done so. 

On April 26, 1913, Ewert placed four potted plants each of Riles 
nigrum, R. aureum, and R. rubrum (var. Red Holland) around a tree 
of Finns strobus heavily infected with Cronartium ribicola. On 
April 26, May 9, 17, and 24, June 6 and 21, and August 3, the several 
plants of each species were treated as follows : 

Plant 1, sprayed with 1 per cent Bordeaux mixture on only the upper surfaces of 
the leaves. 

Plant 2, sprayed with 1 per cent Bordeaux mixture on only the lower surfaces of 
the leaves. 

Plant 3, sprayed with 1 per cent Bordeaux mixture on both surfaces of the leaves. 

Plant 4, untreated check plant. 

The checks on May 17 had two leaves with a considerable number 
of uredinia; on May 25 almost all leaves bore uredinia; and on July 
28 there were 50 heavily infected leaves. 

Plants numbered 1 (sprayed on the upper surfaces only) on May 
25, showed the first uredinia on one leaf; on June 3 six leaves were 
infected, two very heavily; on July 28 there were 50 infected leaves. 

Plants numbered 2 (sprayed on the lower surfaces only) on June 
3 first showed very slight infection on two leaves; on July 28 four 
leaves were infected, all lightly. 

Plants numbered 3 (sprayed on both upper and lower surfaces) on 
July 24 were healthy. On June 28 one leaf bore a single uredinium. 

Here again Ewert fails to state definitely whether or not the 
infections were all on the lower surface of the leaves. 

Ewert 's experiment of 1912, spraying one-half of a Ribes nigrum 
bush, was repeated in 1913. In this case the sprayed leaves re- 
mained healthy, except where they were not thoroughly reached 
with the spray. This exception seems to the writer to be signifi- 
cant, as it indicates that spraying carefully enough to control the 
disease was apparently not practicable even for as painstaking an 
experimenter as Ewert showed himself to be in planning and carry- 
ing out these tests. In 1912 the disease was very virulent, while 
in 1913 it was not. This probably largely explains the better show- 
ing made in 1913 in these experiments. Although spraying greatly 



WHITE-PINE BLISTER RUST. 79 

reduced the number of uredinia, it did not entirely prevent their 
formation. 

The spraying of pines with fungicides apparently has not received 
much attention. It is reported that the infection of young seedlings 
of white pine has been controlled by spraying in Belgium (106). 

Pines have been treated in Europe by the application of various 
chemicals, but the following cases are the only ones where results 
are given. 

Hostermann (55) treated the affected parts of two Pinus strobus 
trees with 10 per cent and 20 per cent solutions of carbolineum. 
This was applied with a brush on April 28, 1908, before the secia 
matured, and again on May 12 and 18. The next spring, secia started 
to develop, and the treatment was repeated with a 50 per cent solu- 
tion of carbolineum. The tree was apparently unhurt, but in the 
spring of 1910 the fungus was still alive. 

In another case (102), where 15-year-old trees were badly attacked 
on the trunk and the leaves had turned noticeably yellow, the bark 
was scraped off and the area bandaged with " carbolineum avena- 
rius." A second tree was scraped and a 10 per cent solution of 
potassium permanganate applied. On a third tree a 1 per cent 
solution of copper sulphate was used. The first and second trees 
recovered and the last one died. 

Buttner (9) treated 18-year-old Pinus monticola trees which had 
trunk infections. Attempts at cutting out the infections failed. 
He then applied u tree wax" to the visible infection itself and 20 cm. 
above and below it. The trees showed no blister rust in 1906. It 
would be interesting to learn if this held true for several years. 

Eriksson (33) recommends the use of tar to cover the infections 
and prevent the distribution of the spores. 

Tubeuf (164) says that valuable trees may be saved by cutting 
out infections and treating with lysol, asphalt, etc., if Bibes are 
removed for a distance of 50 meters, so that no new infection can 
occur. 

Kneiff (74) removed blisters by frequent wet rubbing. He also 
used "tree wax" and cloths wet with carbolineum. These hindered 
the disease, but he says the best way to fight it is the removal and 
burning of the diseased parts or plants. 

Pechon (105) advises burning affected trees and states that treat- 
ment with tar and similar substances will not suffice. 

Kohler (75) tried cutting out and smearing, but gave up these 
methods as causing too much injury and even death. He sprayed 
the trunks with a strong stream of water before the blisters opened 
in the spring. The blisters disappeared and the trees formed new 
bark. 



80 BULLETIN 957, U. S. DEPARTMENT OE AGRICULTURE. 

Fungi which parasitize Cronartium ribicola are not uncommon. 
Their use in artificial inoculation of infections on pines has been 
attempted (172, 173, 174), but with little success. There appears 
to be little prospect for success commensurate with the expense 
involved. 

The separation of the pines from the Bibes plants is the most 
efficient method of controlling the disease in a given locality. To 
judge from the frequency of this recommendation for combating 
the disease in European literature, apparently considerable work of 
this kind has been done in Europe; but no definite statement of 
results in specific instances have been found. 

The use of screens of another species of tree between Bibes and 
Pinus strobus has been recommended in Europe (131, p. 41). No 
one has stated the results of such treatment in any given instance, 
however. 

There must be a chance to secure much valuable data on the 
success or failure of various methods of treatment which have been 
tested in Europe, but which have never been published. This can 
only be done by making definite investigations in Europe from the 
American standpoint. It must be remembered that from the 
European point of view the white pine is an introduced and com- 
paratively unimportant tree. Its diseases, therefore, are not made 
the subject of systematic and prolonged study. Many facts of value 
fundamental to the control of this disease in America can only be 
determined by the intimate study on the ground of the much older 
infections of Europe. 

Experiments in Control in North America. 

In all control of parasitic plant diseases the fundamental thing is 
to determine the extent and the distribution of the disease to be 
controlled. The parasitic fungi are so generally distributed by the 
wind and are so insidious in their spread that they usually have 
gotten well started before their presence is discovered. Newly dis- 
covered imported diseases must be attacked at once or not at all, 
if eradication is to be accomplished, but more attention should be 
given to the matter of determining reliably the extent of outbreaks 
of such diseases. Scouting is a very important part of any disease 
eradication or control campaign.- A well-conducted, intensive, 
plant-disease survey will do much to aid in determining the status 
of a new disease. 

METHODS USED. 

The control of white-pine blister rust has been attempted in 
North America (1) by means of quarantines of the host plants, 
(2) by the eradication of advance infections, (3) by the separation 
of the two hosts, (4) by sanitation, (5) by screening Bibes or pines 



WHITE-PINE BL.LSTEI! RUST. 



81 



with other species, (6) by the judicious selection of planting sites for 
pines, and (7) by such minor methods as spraying, close pasturing 
of Ribes, and the removal of the diseased plants or parts of them. 

QUARANTINE. 

In North America (131, p. 54-55), Canada took the first official 
action against the white-pine blister rust, placing it on her list of 
proscribed plant diseases and later prohibiting the entry of all 
5-leaved pines from 
all other countries. 
Since then (2) a quar- 
antine has been de- 
clared against the 
shipment of Ribes 
from points east of a 
line between Saskat- 
chewan and Alberta 
to points west of that 
line. The shipment 
of Ribes to points 
west of this line is 
allowed from points 
in the United States 
south of the above 
protected area. 
These modifications 
are made to help 
protect the western 
white-pine area from 
the shipment of this 
disease in nursery 
stock, and to connect 
with the Mississippi 
Valley quarantine 
line in the United 
States, which has 
been established 




Fig. 13.— Outline map of North America, showing the quarantine 
lines established by the United States Department of Agriculture to 
control the white-pine blister rust by prohibiting the shipment of the 
host plants from infected territory to uninfected sections. The 
quarantine line established by Canada to prevent the shipment of 
diseased nursery stock across the prairie region from the eastern 
Provinces is also shown. 



with this end in view (fig. 13). The United States Government in 
1912 (94) put in force a regulatory act controlling the entry and 
movement of nursery stock. This act prohibits the entry of 5-leaved 
pines and of Ribes from Europe, Asia, and Canada; forbids the 
shipment of such stock from the eastern section of the country to 
points west of the western boundaries of the States of Minnesota, 
Iowa, Missouri, Arkansas, and Louisiana; and also forbids the 

46103°— 21— Bull. 957 6 



82 BULLETIN 957, U. S. DEPARTMENT OE AGRICULTURE. 

shipment of 5-leaved pines and of Ribes nigrum from the States 
of New England to any of the other States and from New York to 
points outside that State. Still more recently, an absolute embargo 
has been placed on ornamental and forest tree and shrub stock from 
other countries (Hg. 13). 

These quarantines prevent our getting more of the white-pine 
blister rust from other countries. The Great Plains region forms a 
natural barrier (fig. 13) against the spread of this disease from the 
East to the West (97, 98, 141, p. 7; 148)). Since it is already well 
distributed and established east of this barrier, the immensely 
valuable western white pines can be protected very efficiently by 
preventing the shipment of white pines and Ribes from the infected 
section to the western region, which is still free from the disease. 
This is accomplished by quarantine, which is designed to prevent the 
shipment of infected stock from a generally infected district to those 
States which are not generally infected and to exclude plant pests 
from all the rest of the world (98). 

Within the past four years many of the various States have 
enforced regulatory measures with reference to this disease (94). 
These States are California, Delaware, Georgia, Idaho, Illinois, Indi- 
ana, Kansas, Maine, Maryland, Massachusetts, Michigan, Minnesota, 
Montana, Nevada, New Hampshire, New Jersey, New York, North 
Carolina, Oregon, Pennsylvania, Rhode Island, South Carolina, 
South Dakota, Tennessee, Vermont, Washington, West Virginia, 
and Wisconsin. 

ERADICATION OF ADVANCE INFECTIONS. 

In 1909, when Cronartiwn ribicola was first found upon white pines 
in North America, it appeared to occur only on recent shipments of 
young trees from Europe. That is, it was present in advance infec- 
tions, and so far as could be determined there was no generally 
infected area. Since that time areas have been found which are 
generally infected, and we have both types of infections to reckon 
with. (See figs. 2 to 12.) Where advance infections were small it 
appeared to be feasible to attempt eradication of the disease, but 
when generally infected areas were found eradication became impos- 
sible, and local control was the only feasible thing to be attempted. 

When this disease was first discovered on Ribes in 1906 at the 
Agricultural Experiment Station at Geneva, N. Y., an attempt was 
very properly made to eradicate the disease. All of the Ribes in the 
infected plat were destroyed. Very few white pines were within 
half a mile, and none of these were found diseased. Stewart (150) 
published an excellent account of this case. It was not then known 
that the seciospores readily blow for 'miles in a viable condition, nor 
was that fact established until rather recently (128, 145, 146). 



WHITE-PINE BLISTER RUST. 83 

In 1909 it was learned that great quantities of infected young white 
pines had been imported from Europe in 1907, 1908, and 1909. 
(Fig. 1.) With conditions as they appeared to be, it was believed 
that eradication might be possible, and this was attempted. The 
disease was held in check in such shipments of diseased trees as 
could be located. But many could not be located. Moreover, for 
years before, as was subsequently learned, nurserymen and private 
individuals had imported from Europe many infected white pines. 
These we had no means of knowing about until too late, since the 
importers and planters did not inform us concerning them, even after 
the publication of warnings against the disease. Such diseased im- 
portations have been the center s from which most of our large out- 
breaks have started. So far as we can learn no Federal agency has 
imported white pines upon which this disease has been found. 

More complete knowledge of the life history of the fungus has shown 
that it is impossible to eradicate it where both Bibes and white pines 
are native and abundant, after the seciospores are once set free in 
quantity. If both pines and Bibes be removed from a given area 
the disease may be eradicated in that area but it will have escaped 
beyond that area by means of the seciospores. This happened in 
Minnesota and Wisconsin, where all the white pines and Kibes were 
removed from large infected areas. 

The removal of pines has been accomplished in a few cases. Entire 
plantings of imported pine stock were destroyed soon after they were 
found to be diseased, and in these cases Bibes were also removed or 
were absent from the area treated. The forestry officials of the State 
of New York took the lead in this work, destroying 1,200,000 imported 
trees in their nurseries in 1910 and 1911. A number of plantations 
were also destroyed in New York, New Jersey, and Vermont (131, 139). 
As early as 1912, the total destruction of diseased lots of imported 
white pines (133) was urged rather than weeding out only those 
which were visibly diseased. Public opinion would not permit this 
to be done in the wholesale manner that was necessary for efficiency. 
Yet this was the one efficient manner of handling such imported trees 
(136) before generally infected areas had developed. 

SEPARATION OF THE TWO HOSTS. 

The fact that each form of spore will infect but one of the hosts, at 
once indicates that a separation of these hosts will prevent the further 
progress of the parasite within the control area. If the pines only 
are removed, the disease will be likely to die out on the Bibes, since 
it apparently overwinters on them only infrequently; if the Bibes 
are removed, the disease is isolated on that particular lot of pines, 
where it overwinters (if the diseased trees are not also found and 
removed) and produces new crops of geciospores each spring. The 



84 BULLETIN 957, U. S. DEPARTMENT OF AGRICULTURE. 

disease is kept from attacking more pines within the area where the 
Bibes are removed, but it may spread to Ribes miles away, there to 
start new pine infections, each of which will act as a new center of in- 
fection in future years. This makes it practically impossible where 
both white pines and Ribes are native entirely to eradicate the disease 
after the aeciospores are once set free. 

REMOVAL OP PINES. 

In all work where pines have been removed the Ribes have been 
absent or were also removed; hence, all this work is placed under 
" Eradication of advance infections." In some of the States, Ribes- 
growing sections are being established, and it is expected that 
white pines will be entirely removed from such areas. 

REMOVAL OP RIBES. 

Experiments on a large scale are in progress in all of the States of 
New England and in New York, Wisconsin, and Minnesota for the 
removal of all Ribes in certain areas, to determine whether it is 
practicable thus to protect valuable white-pine stands. 

Much work of this sort has been carried on during the past four 
years. Infected areas have been chosen and the Ribes removed 
under various conditions to show what possibilities there are in 
such work (23, 24, 25, 103, 104). The removal of all Ribes plants 
in a given area is a difficult matter. Wild Ribes offer the greatest 
difficulties. It is not humanly possible to find every plant in wild 
woodland; plants pulled up, if left touching the soil, may again take 
root and persist in a living condition; pieces of root crown and oc- 
casional pieces of roots left in the soil sprout and make new plants 
(23, p. 8) ; fruits on the pulled bushes fall off and start a crop of 
seedlings to replace the parent plant; seeds of old fruits already on 
the ground may germinate and start seedlings. Nevertheless, the 
results of this work are encouraging. Wild currants and goose- 
berries do not reproduce rapidly in an area that has been worked 
by an efficient crew. Thorough checking on 2,485 acres in 8 separate 
tracts previously gone over by eradication crews showed that on an 
average acre, 62 bushes (95.5 per cent) were destroyed in the first 
working and 3 bushes in the second working. Of the latter, two 
bushes were missed in the first working and one bush developed 
from seeds or sprouts. The remaining plants are so small that they 
carry but 1 to 2 per cent of the total Ribes leafage (24), Moreover, 
they are usually so low or so covered with other vegetation that very 
few become infected, so that the work results in almost perfect con- 
trol of the disease. To judge from data at hand, control areas usually 
should be reworked within 10 years after the first working (25). 

Two principal methods of removing Ribes have been developed. 
Where Ribes are abundant the Ribes eradication crew has to cover 



WHITE-PINE BLISTER RUST. 85 

all the ground and pull the Ribes as they go. Where Ribes are 
relatively scarce they are likely to occur only in certain favorable 
locations. In such territory an expert scout covers the ground, 
mapping it and indicating the Ribes areas to be worked by the crew. 
In favorable localities this has proved successful and greatly reduces 
the cost of Ribes eradication (25) . Experiments in the killing of 
thick stands of wild Ribes with chemicals indicate that this method 
(113) materially reduces the cost. 

Ribes eradication was started as early as 1909, but at that time 
was limited to plantations of infected imported white pines and to 
a safety belt of 100 yards around them. In 1910, the width of the 
safety belt was increased in some of the States to 500 feet, and in 
1915 in Massachusetts to 500 yards. In 1916, 600 yards was taken 
as the safe width for all situations. Prior to 1919, facts concerning 
the spread of Cronartium ribicola from Ribes to pines were not 
definitely known. As a result of the investigations of the germina- 
tion and dissemination of the sporidia the width of the Ribes-free 
zone was set, in 1919, at 200 to 300 yards for average conditions 
(25, 146). 

In 1917, wherf extensive areas were first cleared of all Ribes, lack 
of experience in such work by all connected with the work greatly 
reduced its efficiency, but even then it was found that the outlook 
was not hopeless, although the cost of eradication of Ribes was too 
high to be justified except where pine stands were valuable. Effi- 
ciency has been steadily increased since then until it has been found 
that men green in this work can be quickly taught to find and destroy 
at least 95 per cent of all wild Ribes plants the first time over the 
ground (24, 25) . A system of checking the work has been developed, 
as well as a system of accounts, so that present results are quite ac- 
curately known. 

The cost of Ribes eradication has been steadily reduced. In 1918, 
105,977 acres were worked in New England at a labor cost of 44 cents 
per acre. In New England and New York the average cost per acre 
including supervision in 1918, was 66 cents. In 1919, in New York 
and New England, 252,114 acres were worked at an average cost per 
acre of 54 cents, including supervision, and of 42 cents for actual 
labor (24, 25). Improved methods are expected to reduce still 
further this cost as, in New England alone, the cost in 1919 was 24 
cents per acre, owing to the use of improved methods (24). 

The efficiency of Ribes eradication with respect to pine infection 
will become evident as time elapses. Examination of areas where 
Ribes were eradicated in 1916 and 1917 has shown no new pine 
infections. This is in spite of the considerable number of Ribes 
missed in the early work. 



86 BULLETIN 957, U. S. DEPARTMENT OF AGRICULTURE. 

The experiments in the removal of Ribes on an extensive scale 
have gone far enough to prove that it is a practical method of pro- 
tecting pine stands. Accordingly the public has been urged to des- 
troy wild and cultivated Ribes within at least 200 yards of valuable 
pine stands in the generally infected regions. 

Assistance to individual pine owners, towns, and associations in protecting pine 
areas from the blister rust is given by the New England States, New York, Wisconsin, 
and Minnesota, in cooperation with the United States Department of Agriculture. 
In 1919, about $10,000 was subscribed for cooperative eradication of currants and goose- 
berries by individuals and associations in New York. In Massachusetts, local coop- 
erators furnished $1,075, In New Hampshire, 53 towns voted appropriations totaling 
$8,514, and 34 individuals and firms subscribed $2,053 additional. The interest of the 
public in blister-rust control is further evidenced by the fact that this State destroyed 
21,171 bushes of cultivated currants and gooseberries belonging to 1,023 owners, and 
only 3 owners insisted on compensation for their bushes (24). 

The State and Federal authorities favor cooperation with towns, 
counties, associations, or groups of individuals in order to free from 
Ribes as large an area as possible in each locality. This reduces cost 
per acre and increases the effectiveness of the protection to pines. 

SCREENING RIBES AND PINES WITH OTHER SPECIES. 

Screens of heavy underbrush or trees surrounding or covering 
Ribes will do much to prevent infection of the Ribes by seciospores 
and will greatly reduce infection of white pines, if the Ribes do be- 
come infected (145, 146). 

Screens or windbreaks of other kinds of trees around the edge of 
white-pine stands will greatly reduce infection on the pines. In the 
same way, planting in mixture or in strips alternating with another 
species should help to keep the disease down. 

SELECTION OF SITES FOR PINES. 

Infection by Cronartium rilicola may be reduced to a rninimum in a 
generally infected area by judiciously choosing a site for the planting 
or natural seeding of white pines. Areas where there is a minimum 
number of Ribes, or where they have been eradicated, which are not 
in moist locations and are not especially subject to fogs or mists, and 
which are protected by forests or conformations of the land from 
heavy sweeping winds, are favorable for the encouragement of the 
growth of white pines. The converse conditions are to be avoided 
as far as possible. 

SPRAYING. 

Spraying has been little used in North America to control the white- 
pine blister rust, as the chances for success have appeared to be 
slight. Spraying of Ribes to prevent their infection has been tested 
in a number of instances. 

In 1915, McCubbin (86) carried on some careful spraying tests with 
Ribes nigrum plants, using both Bordeaux mixture and soluble sul- 



WHITE-PINE BLISTER RUST. 87 

phur in parallel experiments. The following statement has reference 
to spraying that was done every two weeks through the summer : 

It was realized that the spray would have to be applied to the under sides of the 
leaves to be effective, and though this was done as thoroughly as possible in our work, it 
must be admitted that it takes so much time and care that satisfactory spraying of this 
kind would be out of the question in a commercial way. Owing to frequent rains dur- 
ing the summer, the best results were not obtained from this work, but even allowing 
for this it was certain that, though the rust can be greatly reduced by spraying, it can 
not be controlled sufficiently to prevent the spread of infection. Consequently, what- 
ever value spraying methods may have as a means of protecting individual planta- 
tions, they are likely to be of little use in combating the disease as a national pest. In 
this connection, it has been suggested by the Dominion Botanist that since spraying 
will not completely control the rust, it may work a positive harm by keeping the in- 
fected leaves longer on the bushes in the fall, and thus materially extend the period 
during which infection of the pine may take place, providing, of course, that the in- 
fection of pines is possible throughout the whole season. 

Stoddard, in 1918 (23), carried out a spraying test in Connecticut, 
with the following results : 

Spraying experiments for the control of the blister rust were conducted on red and 
black currants. Results were nearly negative on red currants because of lack of in- 
fection. On black currants spraying gave nearly complete control throughout the 
season. However, such careful and frequent spraying had to be done that it is not 
considered to be a practical method of control. 

No experiments have been made in the spraying of pines, as it has 
appeared useless in larger trees where the infections have occurred. 
Seedlings in seed beds of nurseries may perhaps be protected from in- 
fection by spraying, as has been suggested by Clinton and McCormick 
(14) . It should be well tested under extreme conditions (106) . 

CLOSE PASTURING OF RIBES. 

The use of animals to feed on the leaves of Kibes is feasible if the 
area is pastured heavily until well cleaned up. Sheep are very close 
feeders and undoubtedly can be thus utilized (23, p. 7). Goats are 
the most promising animals for the purpose, however, as they are 
omnivorous feeders. This method can be recommended only for 
areas where small pines are absent or too few to be of value. 

REMOVAL OF DISEASED . PARTS AND DISEASED PLANTS OF RffiES. 

The removal of the infected leaves has been attempted in a few 
lots of Ribes nigrum in nurseries, but it is costly and merely palliative 
in that it is usually only postponing more drastic measures. The 
cutting back of infected Ribes bushes has been tested in a few 
instances, but like the plucking of the leaves, it is usually unsatis- 
factory, since the bush remains to take infection another season. 

The removal of diseased plants only of Ribes is unsatisfactory, 
as it has been found that it requires repeated visits through the season 
for the removal of plants which have developed the disease since 



88 BULLETIN 957, U. S. DEPARTMENT OF AGRICULTURE. 

previous inspections. This makes the cost of the work prohibitive, 
and the disease progresses in spite of it. 

REMOVAL OF DISEASED PARTS AND DISEASED PLANTS OF PINES. 

The removal of diseased parts of infected pines does not appear to 
be an economical procedure from the viewpoint of the lumberman or 
wood-lot owner, because of the low value of single trees. For highly 
valued ornamental trees it becomes possible financially. Under such 
conditions, the removal of all Bibes to prevent new infecton, accom- 
panied by careful cutting out of all infections in the pines for several 
years, will finally result in the elimination of the disease from those 
trees. Martin, Gravatt, and Posey 58 have investigated the possi- 
bilities of this type of work. They conclude that — 

Experimental and practical results show that ornamental pines which have already 
become diseased can be saved by cutting out the infected parts if treatment is applied 
in time. The work is easily performed at a comparatively low cost. Treatment can 
be given any time during the year, but best results will be obtained from April to June 
when the cankers are more easily found because of the bright orange-yellow blisters. 
Successful treatment depends primarily upon ability to find the cankers and deter- 
mine accurately the edge of the diseased area. The workmen should be thoroughly 
familiar with the symptoms and appearance of the disease on pines. 

Where the Bibes can not be thus eradicated, other species of trees 
should be planted to take the place of the white pines. Cutting out 
infections depends, for success, on finding all of them. The work- 
man must be familiar with the blister rust and be thorough or the 
results will not justify the cost. If a tree is nearly girdled near the 
ground, or if most of the branches must be removed, it is useless to 
attempt to save it. The cutting out must be repeated for several 
years after all the Bibes are eradicated, as dormant and slightly 
developed infections become visible. Cutting out experiments 
showed that cutting back for 1^ inches or more from the extreme 
edge of the infected area insured removal of all diseased bark and stop- 
ped the disease in those areas. In practice this distance should be 
increased to 5 or 6 inches to insure thorough work. 

Oh. a main trunk an infection which has extended only part way 
around the trunk may be treated by peeling off the bark on the canker 
and to the required distance around it. In this case the safety zone 
should extend for 4 or 5 inches directly above and below the diseased 
area but need not extend more than half as far sidewise. 

The removal of only diseased white pines in infected imported 
trees has been inefficient and costly, even where the Bibes were 
removed too. Becords of such work in about 900,000 imported trees 
(136) shows that it is inefficient, although the disease has been checked 
somewhat. The cost has been great enough to have replaced the 
imported trees with healthy home-grown ones. 

58 Martin, J. F., Gravatt, G. F., and Posey, G. B. Treatment of ornamental white pines infected with 
blister rust. U. S. Dept. Agr. Cir. 177, 20 p., 12 fig. 1921. Seen in manuscript. 



WHITE-PINE BLISTEB RUST. 89 

In a region where Ribes are rare or practically absent the removal 
of the diseased pines only may serve to prevent progress of the disease. 
This allows the dispersal of seciospores and may result in scattering 
infection on Ribes miles away. In such cases it will take a long time 
to detect the escape of the fungus. In areas free from the Ribes the 
aim should be eradication rather than control of the disease, as such 
areas are the very ones where white pines should be grown in the 
future. 

In generally infected districts where Ribes are removed for some 
distance, it may pay to cut out the worst infected trees to reduce the 
crop of seciospores and thus reduce infection around the borders of 
the treated area. 

Status of the Control of White-Pine Blister Rust. 

The present status of the control of the white-pine blister rust in 
North America may be summed up as follows : 

Eradication of Cronartium ribicola is impossible except in small, isolated, advance 
infections. It should be attempted only in localities where the disease is quite lim- 
ited in distribution and well separated from the known generally infected areas shown 
in figure 2. As a national problem, control is the only feasible thing. Protection of 
uninfected or sparsely infected areas by enforcement of the present Federal quaran- 
tines is necessary, since this disease is distributed to great distances only by means of 
infected nursery stock. The western forests of white pines can be protected from the 
blister rust for an indefinite period by rigid enforcement of the Mississippi Valley 
quarantine. A single diseased shipment may undo all attempts to restrict it to the 
eastern forests. 

In the eastern forests blister-rust infection on Pinus strobus is 
rapidly developing. A strip survey in one locality in New Hampshire 
(24) shows that one-fourth of the white pines on an area of 72 square 
miles are now infected. The areas marked as generally infected in 
figure 2 show the great increase in general pine infection. Much of 
this infection will become visible in the next three years. It is an 
insidious disease, a tree not being noticed as diseased until it is 
heavily infected. There is abundant evidence that it is destructive 
to merchantable trees as well as to younger ones. It is just getting 
under headway. 

Ribes nigrum is far the most dangerous species, but all Ribes are 
dangerous to white pines in generally infected areas. In such areas 
the disease can be controlled by the removal of all Ribes. Local con- 
trol depends on the removal of Ribes within white-pine areas and 
the education of the white-pine owners to remove Ribes as a routine 
part of white-pine forest management. Local control by the removal 
of Ribes can be taken up at any time in the future, but if the present 
stand of trees is to be saved action must be taken at once. 



90 BULLETIN 957, TJ. S. DEPARTMENT OF AGRICULTURE. 

A few years delay will mean serious loss. It is a simple and practical precaution to 
destroy the currant and gooseberry bushes before they destroy the pines. The demon- 
strated effectiveness of this method of control justifies pine owners in uprooting currant 
and gooseberry bushes on a large scale (24). 

Those who do not do this in their pine lands and for a distance of 
200 to 300 yards outside will be likely to see a valuable asset turned 
into' a liability. In areas where the white pine is an important tree 
cultivated Bibes should not be planted. A number of States already 
have laws prohibiting such planting without permission from the 
State authorities except in areas designated and set apart as " cur- 
rant-growing districts." 

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WHITE-PINE BLISTER RUST. 91 

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